The only point of genres is to serve as a label so that other people know what the hell you're talking about. That's it. If I tell someone that I'm reading a science fiction book, watching a sword 'n sorcery movie, or playing a run 'n gun game on the Genesis that conjures up certain common (with slight differences) images in everyone. It helps facilitate conversation. Genre definitions aren't created by academics or writers, but by popular use until they become a de facto standard. This mean that genre definitions will change over time as new fans and subgenres enter the equation.
So genres enable people to talk about books, pick which books they like, and market their books appropriately... but they're utterly meaningless?
What else would you expect them to do?