1.To feel pain on the outside instead of the inside
2.To cope with feelings
3.To express anger towards themselves
4.To feel alive and real
5.To feel the rush
6.To help with focus and concentration
6. And yes, sometimes for attention, but that is usually only for seeking help when they don't know things are wrong and they don't feel right but they don't know how to make others understand or how else to get help.

And unlike an apparent misconception, most of us don't go instantly tell our best friend. We hide it, and hide it, and hide it some more, until we finally feel like we are going to burst under the strain of our secret shame so we tell our best friend who we think we can confide in and who might be able to help us get under control. Then we often get to watch the relationship disintegrate as they let all of there ignorance show, leave us because they think the problem will go away or because they can't handle it, or because they think it is attention seeking and if they ignore it we will eventually stop doing it.

Hope that helps you understand a little better