Anonymous cash on the internet. Nobody knows who owns each bitcoin account so you can do all sorts of things you could not otherwise. It's very similar to (for example) Pecunix, although unlike Pecunix it's not backed by gold and it's distributed which makes it hard to take down. Anything black or greymarket is suitable for bitcoin.

100% legitimate uses? Well you could use it instead of Paypal and save a couple of bucks in fees but it's a lot harder than just using Paypal and there's no recourse if something goes wrong.