Many things. As with most cartoons, the creators will often sneak references to their favorite movies, tv shows, books, etc... into the story/cartoon which a bunch of people can identify with. Also people like to be different. I suppose a lot of people watch the show for the same reason people like anything weird and not the norm - not necessarily because they like the thing itself, because they feel that the thing will help make them unique or different from all the other "cogs in the machine" and they enjoy the community they become apart of.
Honestly, I can't find a reason why anyone would seriously like the cartoon except in rare cases (sometimes people just like things for no real reason other than they just do) and I feel that a majority of the "fandom" is just there because they want to be apart of that fandom for various reasons (a group of friends, wanting to fit in with some sort of group, etc...)
