There are a few reasons that pop up a lot...
Here are some recurring themes in the autobiographies of famous people:
Set the record straight Bill Mauldin, the great WWII (and after) editorial cartoonist, wrote two autobiographies: Up Front, basically a press release for the war effort and his cartoons, and The Brass Ring, where he dished some dirt and did not care about looking like a good or smart individual.
I'm not as bad as they say I am Lenny Bruce's How to Talk Dirty and Influence People was an attempt to rein in his image as a foul-mouthed showbiz liability. He makes himself seem pretty reasonable, without denying anything that wasn't already common knowledge. Unfortunately, he died of a heroin overdose shortly after it was released.
Okay, I'm as bad as they say I am, but I did some good stuff, too Most Rock n Roll musicians know better than to claim they were innocent saints. Gregg Allman and Chuck Berry wrote autobiographies of the "warts and all" variety.
I'm promoting a new project Some of the most entertaining autobiographies are by actors or comedians who are less interested in defending their reputations than in ginning up interest in an upcoming show or project. Trevor Noah's Born a Crime: Stories from a South African Childhood buoyed interest in his television show, and Steve Martin's Born Standing Up functioned as a bridge between his changing his focus from comedy to music.
Please vote for me Barack Obama's Dreams From my Father and The Audacity of Hope were instrumental in selling him to a skeptical public as a serious presidential contender.