When you exercise, you use up a lot of hydrating electrolytes, carbs, and important vitamins in your body. Naturally, you'd have to replenish your yourself by drinking Gatorade and eating fruits. On the other hand, beer also has sugary carbs, a hint of electrolytes, and a few helpful plant-based nutrients that come from the hops, yeast, and barley--all of which are good for replenishment.
So what's wrong with beer?
Beer has alcohol. Alcohol dehydrates the body and hinders muscle recovery. In other words, it screws up your progress and makes your workout useless.