Side spin is crucial for my game at least, because normally I'm slicing DTL on my backhand in the Ad court. This means that when I hit the line, it's drawing my opponent into the doubles alley and opening up a ton of court space. This is my bread and butter approach shot as it just works really well for me.
Sidespin on the forehand I don't hit ever when I'm hitting a normal rally ball, that being said when I'm on the run its almost always a buggy whip shot that has a ton of action on it from the low to high and across motion. It's just physics at that point, (which I don't know very well in depth!) that with enough racket head speed I'm going to be getting a lot of spin.