There is a serious problem with determining the exact benefits of vegetarianism, not because there are none, but because there hasn't been any serious widespread scientific research in this area. Depending on the pre-assumptions of the researchers, the results vary from “you'll die (painfully) if you ever try to quit eating meat” to “eat fruits only, everything else is a poison”. However, it is clear that there are several undisputed benefits of vegetarianism – admitted even by those who rally against it.
Lower BMI
As most vegetarian diets are low-fat ones (average vegetarian eats 25% fat less than a typical meat-eater), they usually effect in the lower Body Mass Index of an average vegetarian. This is probably the most visible of all benefits of vegetarianism and the main reason for many people to become a vegetarian.
Lower cholesterol level
Other widely noted benefits of vegetarianism is decreasing the cholesterol level (by 0.5 mmol/l) and subsequently, the decreased chance of suffering from a heart attack. However, due to the complexity of the heart attack issue, it is still sometimes argued that such decrease results from other factors than a vegetarian diet.
Longer average life expectancy
One of the best-known benefits of vegetarianism is, however, not the low probability of having a heart attack, but the longer life expectancy. Vegetarians, on average, live a little longer than the rest of us. Whatever benefits and disadvantages vegetarian diets have, it seems that they give us a little more than they take away.
Source: http://www.benefitsofvegetarianism.com/