You don’t have to be an experienced cat watcher to know that cats can sleep darn near anywhere, your bed, carpet, tile, linoleum, concrete, gravel, grass, a tree branch, even on top of radiators. Unlike humans, cats do not require a soft (or flat) surface for sleep. In fact, part of the joy in owning a cat is observing all the varied places he chooses to sleep.
Given that obvious fact, why do owners buy special beds for their pets? Go with the obvious answer: cat beds, like numerous other pet products, are made to appeal to the owner, not to any real need of the cat. If you buy your cat a bed, he will probably sleep in it—there, and everywhere else in your home.