It really isn't about just what the cat looks like ... individual breeds normally also possess similar behavioral traits. Sometimes people like to know that they will be getting an extremely active, athletic, vocal, full-of-pi$$-and-vinegar Seal Point Siamese cat because they had one they were particularly fond of as a young person or whatever. Looks are not the only thing that make up a breed.
Exactly. Many people are creatures of habit. Many grow up with a calico, a siamese, a tailless, a big shaggy lovebug of a cat with a tiny voice, etc. and seek that familiarity later in life.
Personally, I've found that I like fluffy (mediumhaired/single coated longhair), laid-back cats who aren't needy (is able to amuse itself whether or not I'm around, but is still affectionate), and aren't very vocal (or have soft voices when they are), and aren't overly dominant with other cats.
If given my pick of a roomful of cats, that is the one I would start out seeking every time.
It's not too different from breed preference/selection.