Creating attention , they usually feel left out or ignored or something deep down them got something to tell but dont dare to tell.
So slash wrist lor , relieve stress (why i oso dunno , still researching) , and create obvious mark so ppl will notice them , and ppl curious ask why hand lai dat , then they will feel being cared by others
The above description is extracted from some books i have read long long ago and what i personally feels , btw , i dun slash wrist , so dun get me wrong that i would do that or ask me how it feel