This Washington Post analysis lays out, in scenario #4, why Trump needs Sessions to quit as Attorney General. The Vacancies Act of 1998 allows the President to appoint another Senate confirmed individual to fill this position if there is a resignation. The Act does not explicitly state a president can take this action if the individual is fired.

https://www.washingtonpost.com/news/politics/wp/2017/07/27/how-rick-perry-could-fire-robert-mueller/