For Both the SAT essay and the ACT writing are considered in the admission process for the UC’s, but they will put more emphasis on your personal insight essays more than the SAT/ACT essays. I would not be surprised that the essay/writing requirement may disappear within the next few years.
My belief is that since the essays on both these exams cannot be coached, they like to see how well applicants are able to articulate their thoughts on the fly. (Again this is my take on why the essays are considered).