Build muscular size which can help prevent injury. Many times I have hit myself on the corner of the table with my leg, and nearly fallen down and cried with the pain. Since I have starte lifting weights and running though, I barely even notice a hard bump.
Endorphins. Best. Drug. Ever. Seriously, running or lifting is hardly ever enjoyable at every instant. Most of the time the feeling of euphoria occurs after a bout of running, or after a certain amount of time of running. When lifting weights the phenomenon known as the pump is a desired feeling. Blood rushing in a muscle invokes a feeling like none other, it simply is amazing.
It does wonders for confidence. Not only do you feel stronger, more flexable and better balanced, you feel like you can take on the world. This can carry over to academic studies or any other area of life. Personally not only do I feel more confident on days I exercise, my brain works ten times better. (seems to work better)
It increases your metabolic rate allowing you to eat more without gaining weight. Buffets are no longer something you HAVE to avoid at all costs. MMMM buffets!
It can help you rest better at night and give you a better quality of sleep. I have found that if my body is tired, not just my mind, I sleep deeper and feel more rested come morning.
It can strengthen your immune system. Of course over exercise can decrease your immune system, but proper exercise and rest/recovery can have all kinds of positive health benifits for your immune system.
Walking up a flight of stairs and being out of breath is embarassing, with proper exercise the cardiovascular system is improved and people will wonder with awe that you just ran up the stairs at a good clip and are not breathing heavily or sweating profusely.
Lowers insulin sensitivity, LDL levels decrease while HDL levels increase, Lowers systolic and distolic (sp?) blood pressure, reduce triglycerides, and even lowers the risk of heart disease.
There are more, which is all the reason to include 30 mins of exercise at least three times a week in any schedule.