Why do some troubled people cut they're arms and wrists on purpose?

[Molly]

Ok, this random girl on twitter followed me. So I checked her account out to see who she is, and her account was disturbing. She had pictures of her arms all cut, but the blood dried. Out of the cuts, it spelled "stop". She wrote as the tweet "The blood dried before my family can see it". That all led up to me wondering WHY people like her cut. Like, how does that help? Having pain in your arms (because they're cut), and having cuts all over your wrists? I've read other questions like this, but everyone else said they want attention, but I don't know why they CUT, out of all things. Why cut? It's useless and won't do u any good. I don't cut but I know that. So why do people cut?