What do rabbits need long ears for?

No, a common misconception. Look up a picture of an "enlgish lop" sometime. Very very long ears (22 inch span). This breed in particular originated in Africa. The ears on any rabbit are to release heat. They are like a built in AC. With so much surface area, the heat given off allows long eared breeds to survive in hotter climates better.

Dwarf rabbits, who's ears are the smallest, are that way to conserve heat. With less surface area the ears don't give off as much heat and allow the rabbit to survive better in colder climates.