An autobiography, if properly written, is a record of your life. It is an insight into the way you thought, the way you acted. It reveals what kind of person you are, for, as Ralph Waldo Emerson said, “Whatever language you use, you will never be anything but what you are.”
An autobiography is a memento, a living legacy of who you are and what you did. It is an opportunity for other people to learn about you. It is an opportunity to spread ideas about whatever you believed in. It is, if written well, you, the person, condensed into a book.
If it sells well, you get royalties. That is somewhat promising.
Also, if you become an important person, people will appreciate your autobiography. For no matter how good a biography can be it will never compare with an autobiography, for who can write about you better than yourself?