People want to leave their legacy behind, I think. People want to be remembered and be able to say that they left their mark in the world. Others want to share the nuggets of wisdom that they obtained through trial and error so that younger generations don't make the same mistakes. As people age, they go through different stages, as we all know. By the time most of life's goals are accomplished and there's nothing left to do, why not give back to future generations. Keeping records like diaries and journals are great ways to preserve our personal histories for our posterity, too. If the public won't read your autobiography, there's a better chance that your posterity will, and they will appreciate it.