Balance is very important for my topspin serves. It's one of the things I look for when I practice my serve. Without changing anything in my motion, having all my momentum moving up at the ball gives me more pace and spin. It's such an easy thing to fix too.
Balance is everywhere in groundstrokes. If you don't split step to unweight yourself, you could be wrong footed if somebody hits behind you. You could hit with your weight on your backfoot, or you could be backpedaling while hitting. Leaning and reaching for balls is bad if you can avoid it. Hitting on the run is all about balance. Not only do you have to hit balanced, but then you have to stop, recover, and move back to the center. Someone who stays low will be able to react faster, move faster, and change directions faster.