There are many capabilities sizzle.js provides that go beyond CSS specifications (and also in some cases that are in the specification, but not yet implemented

There are many capabilities sizzle.js provides that go beyond CSS specifications (and also in some cases that are in the specification, but not yet implemented by browsers). Selectors can get pretty nasty, and having more flexibility available can help to make them more manageable. A lot of times you wouldn't want to use the more complex selectors, but it's certainly nice to have them.