I've been using vim for a decade and I'm trying to switch to emacs. Retaining muscle memory is only possible if I keep using the bindings I learn. Remembering all of them is difficult, and using a subset will lead to suboptimal results.
Some kind of app that quizzes me seemed like a good path to learning emacs, since after 100 hours I would end up much more effective. Then I realized maybe an app like that might be helpful for anyone investing 100 hours into a variety of things.