1) Sweat is the body’s way of cooling itself. If your homeostatic thermostat had a fan, it would be blowing hot air out and pushing cool air in. The same thing happens with sweating: although sweat may smell bad, nasty things like bacteria don’t live on your skin and can’t survive if they’re not there to begin with!
2) Sweating helps you break down fat for energy (see #3). You can also replace electrolytes that are lost through sweat by drinking something like Gatorade or PowerAde.
3) When you sweat, some minerals such as sodium and potassium are absorbed into the bloodstream; these minerals help regulate blood pressure and muscle contraction. For the most part though (especially when exercising), we need more water than we “break” down by breaking down our own food—so we need to drink more fluids during exercise than normal everyday activities!