The answer can vary but possibly it's to justify your behavior, to exact revenge on those who have hurt you, or to crusade for a cause. You might write to warn others or protect them from victimization.
The second question is, What is the theme? Every life has one—or at least it does if you write it.
In 1990, I wrote Norman Vaughan's memoir (which is still in print) called With Byrd at the Bottom of the World. The theme was about a Harvard dropout who wanted to be part of history by driving dog teams on Richard Byrd's historic expedition to the South Pole in 1928–1930.
Norman, who died in 2005 at age 100, was the last surviving member of that expedition. The story has great historic value. (For one thing, they proved that Antarctica is a continent.) The Norwegian explorer Armundson had reached the pole on foot and Byrd was the first to take off from the ice of Antarctica and fly over the pole.
The title also makes it obvious to readers and shows them the limited focus of the book. My second book with Norman was called My Life of Adventure. The title says it was an autobiography.
I think of Russell Baker, who was clear on his purpose in writing his autobiography, Growing Up. On a talk show, he said that children ought to know what it was like to be young in the time before jet planes, super-highways, H-bombs, and the global village of television. He had a purpose and theme and stayed with them.
If I write about my life, I will have a purpose and a theme.