Cleaning removes most of the old dried on yeast, beer, trub, etc., but it doesn't get rid of bacteria, wild yeasts or other microscopic contamination. Cleaning only isn't good enough. Clean and sanitize. Sanitizing will get rid of almost all of the rest of the microscopic organisms. You can't sanitize, unless you first clean though.
You're goal is to give your chosen yeast variety the advantage. When we pitch billions of cells of our chosen yeast, we want them to be competing with the smallest quantity of wild yeasts and bacterias as possible. There is no such thing as sterile wort in the homebrew environment, so our goal should be to give our chosen yeast the best chance to compete against the other stuff.