Different variations for different needs. It's one of the goods points with linux I'd say, you probably wouldn't like the setup I'm using and the same way around. It's really hard to choose, especially for someone new, but the points is there's something that suits everyone or at least you can come much closer than you could ever come with only one to choose.

Some people like a graphic interface while others love the command line and want to do everything there. Some are power users who configure everything to the smallest detail (I've lost count of how many hours I've personally spent on configuring stuff so it's just the way I want) and others simply don't care.

What you must remember is that all distros are just a kernel with various programs, setups and philosophies surrounding it. Some are easy to use but huge and complex, others are bare-bone but simple and there are lot's in the middle, or little of both. You could start with a kernel-only setup or create your own collection of stuff you like, make your own distro.

You can draw similarities to whatever really - Which car should I pick? Why aren't all kitchen tables the same?

Why are there so many linux distros when there's only one windows and mac os you might ask? To begin they are different too, both versions (xp, vista, win 7..) and specialization (professional, home user..) but not all too different. Why? Probably because there's a company promoting, developing and supporting them and it's too hard, expensive and generally unfeasible. Why make two competing products? Let someone else do that, and they have! Unix, Linux, BSD etc.

As someone else said "it's because we can" explains quite a lot. Because someone finds it useful I'd say.

* Sorry it got a bit long winding.