It fills a need. Someone who feels empty inside may cut to feel "something". Someone who is hurting emotionally may cut so that the physical pain will distract them. Some cut to punish themselves. One may cut in the belief that they will let out negativity or evil.
See Below. When people self-mutilate that might experience the rush, "high" similar to the "high" from substances. Oftentimes, self-mutilators experience relief after the act and they're looking for relieve every single time they self-mutilate. The reasons for self-mutilations are different.