They aren't completely necessary, but I find bottom feeders helpful when I feed a little to much to the top dwelling fish. Usually it's the ghost shrimp that take care of the extra flakes and such, and the Cories eat some of the algae. If you're trying to get rid of algae, then a small pleco (depending on tank size) or a school of Ottos would be best. In other words, No. You don't NEED them, but they help to keep the tank clean and healthy.
Are you just wondering if you need them, or looking to them for help with algae, or what?