I'm not the best example for side kicks becuase I can't even throw them right, but they do work.
If it lands good it can hurt the person. It the other guy is one of those speedy guys that moves alot, then it's hard to make it land hard. If it's somebody who's flat footed and just stands there, then you can make it land hard very easy. Or if you time it right they can walk into the side kick. And back kicks are actually spinning side kicks.