Autobiographies are important because it allows you to tell your own story. Only you know what you are thinking, so by writing your own story, you can include thoughts that you did not say out loud.
Biographies are told by a limited point of view, the person who is writing about you cannot tell the audience what you are thinking at the deepest points unless he asks, and even then, he cannot always present your thoughts the way you would like them to be presented.
This distinction can make all the difference in the world. Writing an autobiography allows you to tell a story within a story by showing your deepest emotions.
I would prefer to read an autobiography because you can learn more about someone by reading their own thoughts and not words written by someone else about that person. It seems more personal, like you are in a conversation in which the writer is telling you something.