My take is that it's for the power. Megalomaniacs crave that power over other people. At her age and her net worth, she could retire quietly. Her mental illness compels her forward. She must control the stage, the script, all the players thereon, and all the people who finance the play (especially them). There is no torch to pass on. It must burn in her grubby, grimey hands forever.
Yup, power is fun to Mrs. Bill Clinton.