A lot of busy, smart people have seemingly random side-projects. For example, Von Neumann:
"A professor of Byzantine history at Princeton once said that von Neumann had greater expertise in Byzantine history than he did" [1]
I don't know for sure why, but I think two possibilities are likely: (1) An extremely strong, natural intellectual curiosity and/or (2) Working on other things allows them to bring fresh ideas/insights to their "main" work, and in this sense is also rejuvenating.
"A professor of Byzantine history at Princeton once said that von Neumann had greater expertise in Byzantine history than he did" [1]
I don't know for sure why, but I think two possibilities are likely: (1) An extremely strong, natural intellectual curiosity and/or (2) Working on other things allows them to bring fresh ideas/insights to their "main" work, and in this sense is also rejuvenating.
[1] https://en.m.wikipedia.org/wiki/John_von_Neumann