Many people do this because they don't have the flexibility in their ankels to go down all the way otherwise. It's amazing how much of a difference it makes. I do it myself because I broke my ankle a few years ago and have two screws in it, without anything under my heels I can't get a very good motion, but by raising them just a little bit I can get a full range. There may be other reasons too, but that''s why I do it.