Well seeing as nobody else has my name it shouldn't be too difficult to just have that, although I don't actually want it. So I'll just probably keep trying more and more obscure combinations of rude and silly words until one that hasn't been taken crops up that I can keep and will promptly forget to ever use
You don't have to, but you can. Reason detailed above - so people can give 'nice' URLs to people to find them by, ones that don't include long strings of numbers that mean little to anyone.
But once chosen, you can't change so go to that URL and try and grab something you want now, before someone else does.
Don't see why so many people are having a moan about it. Perhaps you've all got fairly unique names. Whilst mine isn't TOO bad, there's still a lot of Sam Hales out there, and telling someone just to search for Sam Hale would take them about 50 times longer than just going to my username.