Genre-wise, it's mainly a layover from yaoi with the general conception that, since they're both the same sex, one of them apparently has to play "male" and "female" roles, outdated as it is. As far as yuri is concerned, it's more used in terms of character dynamics (eg. the sporty tomboy would be considered the top while the ditzy shy librarian would be considered the bottom). While many people such as yourself probably frown upon that kind of thing, it does create some interesting dynamics. One being role reversal, where the girl who usually bottoms gets on top, and another being playing with your expectations (praise be the Oneeloli subgenre). Of course, you do get some series or doujins where there's no clear top or bottom and it's just two (or more) girls mutually making each other feel good, and those tend to be pretty swell.
