The advantages of plant-based eating are many and include everything from improved mortality rates to potentially short term and long term testosterone. (See my link on Plant-Based Nutrition and Vegetarians and Testosterone for some basic information.) I also eat a plant-based diet, because it the foundation of what I call Orgasmatarianism, eating to maximize nitric oxide and erectile strength and minimizing the chance for future erectile dysfunction.
NOTE: I actually do eat a little meat, which is why I actually prefer the label plant-based. My own dietary regimen includes the consumption of abundant egg whites along with some undenatured whey - what I call the Whey of Life - and nonfat milk.
Eating plant-based has really helped me and I would never go back, but I do have to admit something: there are some nutrients that are predominantly in meats and animal products that have incredible advantages. Examples of this are Vitamin B12, Choline and the subject of this page: creatine.
Many men, especially those involved in sports or bodybuilding, have heard of some of the common advantages of creatine. I would like to quickly cover some of these that, although well-covered in the popular health press, many men may not have heard of them:
CAUTION: Creatine has a lot of nice properties, but may put some men at risk. For example, did you know that one study says creatine raises DHT levels and may put some men at risk who have kidney issues? For more information, see my link on Potential Creatine Dangers.
1. Satellite Cells. One of the most important things you can do for your long term health is maintain your muscle mass over the decades. And, actually, most men will slowly lose muscle mass over the years from poor diet, loss of testosterone and a sedentary lifestyle. To build (and repair) muscle, you need a certain kind of specialized cell called "satellite cells". Testosterone, the amino acid leucine - present in Branched Chain Amino Acids and whey - and IGF-1 are all well-known to the bodybuilding and athletic communities. And all three of these have part of their reputation based on the fact that they increase muscle satellite cell counts.
However, it's not just these big boys that will increase satellite cells: creatine has been shown in a few animal and human studies to do the same thing.  Again, this is absolutely critical for "hypertrophy" or "muscle-building."
2. IGF-1. Everyone has heard of testosterone, but IGF-1 is another hormone absolutely critical for muscle growth and development. IGF-1 and Growth Hormone often go hand-in-hand and have many important properties for maintaining a youthful physique. This is where creatine comes in again: it increases the activity of muscle IGF-1 according to both in vitro and in vivo studies. 
3. Muscle Oxidation. Weight lifting and strength training can put a significant oxidative (free radical) load on the body. And is it any wonder consider that cells are damaged and must be rebuilt? It turns out that creatine actually is highly protective of muscle tissue by increasing oxidative protection. Researchers have discovered that it does this not only by acting as an antioxidant but by stimulating other metabolic antioxidant activities. 
Furthermore, the benefits of creatine supplementation probably increase as you age. The reason is probably most that muscle mass is so cardioprotective as the years go by. As mentioned, most men (and women) slowly lose muscle - about 10 pounds per decade! - and replace it with fat. Let's say that you are one of the few that still weight the same as you do in college. You may be feeling good about yourself not realizing that you have probably lost at least 10 pounds of muscle and replaced it with 10 pounds of fat (unless you are a pretty avid exerciser).
Studies on seniors and the elderly have found one benefit after another from creatine supplementation: muscle building, increased strength, increased fat free mass and so on.  However, one very interesting benefit is improved cognition. One study on seniors found this from dosages of "about 20 g/day for 5 days or about 2 g/day for 30 days." 
These benefits to both young and old are actually remarkable when you consider that there is considerable creatine in meat and most people in modern societies eat a lot of meat. However, to even get a 5 gram dose of creatine would require one to eat 2.5 pounds of raw beef as cooking the beef removes most of the creatine. Fish has more creatine per unit weight, but, again, cooking removes most of it.
Regardless, carnivores have higher muscle creatine levels on average than vegetarians, because, even after cooking, their dietary consumption of creatine is much higher. For this reason, one study that compared vegetarians to carnivores found on average that meat-eaters had about 60% higher plasma creatine levels for example. 
NOTE: One nice benefit of creatine supplement is that it likely lowers homocysteine levels in some populations, at least according to some animal studies. 
And this leads to an important question that researchers decided to study: would vegetarians actually get the most benefit from creatine supplementation, since they tend to get the least amount in their diet? The studies have been somewhat limited, but, so far, the results seem to show that vegetarians would definitely benefit in certain key areas from additional supplemental creatine. Here are a few examples from the research:
1. Cognition. Two studies now have shown that creatine supplementation significantly helps the brain. It all started with a study of male and female vegetarians that showed improvements in both working memory and intelligence from creatine supplementation.  A follow-up study on female vegetarians found that they signficantly improved memory (and choice reaction time in certain ways).  Again, most men think of creatine as helping muscles only, but it profoundly improves mitochodrial function and that, in turn, help the brain and all its heavy processing activities.
2. Exercise Performance. There is evidence that lower creatine levels affect max level exercise performance and that vegetarians would benefit from supplementation. 
3. Muscle Benefits. Vegetarians should get all the muscle benefits of omnivores, because with creatine supplementation, their muscle creatine levels quickly equal that of meat-eaters. This is because muscle can only store so much creatine anyway, so supplementation quickly "levels the playing field". So, if you're plant-based or vegetarian, get ready to "rock" with creatine.
4. Lowering Post-Exercise Inflammation. Creatine does a nice job of lowering both TNF alpha and CRP (C-Reactive Protein) according to one recent study. See #21 on my page on How to Lower Inflammation for more details.
CAUTION: Creatine was fairly recently studied by Brazilian scientists, who monitored kidney function in young men who consumed higher dosages (10 grams/day) of creatine for about 90 days. Their conclusion? Creatine caused absolutely no issues in kideny function. However, a little more caution is definitely in order, perhaps, for middle-aged and beyond men with kidney issues. It is probably wise to drink some extra water when consuming creatine and, of course, talk to your doctor as there have been reports of men with kidney issues having trouble with creatine.
1) The Journal of Physiology, Jun 2006, 573(2):525-534, "Creatine supplementation augments the increase in satellite cell and myonuclei number in human skeletal muscle induced by strength training"
2) EBS Lett, 2004 Jan 16, 557(1-3):243-7, "Creatine increases IGF-I and myogenic regulatory factor mRNA in C(2)C(12) cells"
3) Med Sci Sports Exerc. 2005 May, 37(5):731-6, "Increased IGF mRNA in human skeletal muscle after creatine supplementation"
4) Mol Nutr Food Res, 2009 Sep, 53(9):1187-204, "Creatine supplementation prevents the inhibition of myogenic differentiation in oxidatively injured C2C12 murine myoblasts"
5) http://www.brjb.com.br/files/brjb_128_4201012_id2.pdf, "Benefits of creatine supplementation in older adults"
6) Amino Acids, 2011 May, 40(5):1349-62, "Use of creatine in the elderly and evidence for effects on cognitive function in young and old"
7) Clinical Chemistry, 1989, 35(8), p.1802
8) Kidney International, 2003, 64:1331ï¿½1337; "Creatine supplementation decreases homocysteine in an animal model of uremia"9) Proc. R. Soc. Lond. B, Oct 2003, 270(1529):2147-2150, "Oral creatine monohydrate supplementation improves brain performance: a doubleï¿½blind, placeboï¿½controlled, crossï¿½over trial"
10) Behaviour, Appetite and Obesity, Received February 03 2010, "The influence of creatine supplementation on the cognitive functioning of vegetarians and omnivores"
11) Nutrition, 2004 Jul-Aug, 20(7-8):696-703, "Nutritional considerations for vegetarian athletes"
12) Int J Sport Nutr Exerc Metab, 2002 Sep, 12(3):336-48, "Effect of creatine supplementation and a lacto-ovo-vegetarian diet on muscle creatine concentration"