What is the use of writing autobiography?

An autobiography gives the writer a chance to share their life experiences with others, and while situations and events may not be unique, others can enjoy reading about them, identifying with them, and sometimes even get the comfort of knowing they aren't the only ones who have had similar things happen to them. Some people's life stories are filled with events that ARE unique, and in this case, others can get a glimpse of situations that they would have no other way of knowing about, which can be very interesting.