Someone mentioned above that it can be a way of telling people that the person is hurting when they can't seem to express it otherwise... I can attest to that one.
Also the distraction one is common...
(Incidentally I found self-harm to be almost 'catching' in a way - so many young people in difficult situations and then they see that a friend or someone else cuts and talk to them about it and the friend says it helps... so next time they feel snowed under they try it too...)