The reasoning is that, similar to memorization of times tables, being able to recall a definition or a theorem and its context / assumptions "automatically" wit

The reasoning is that, similar to memorization of times tables, being able to recall a definition or a theorem and its context / assumptions "automatically" without needing to use your brain frees you to worry about higher order activities. Being able to apply a theorem over and over again ultimately builds mastery and internalization. Counterintuitively, mathematicians like being "brain-off" as much as possible -- you want to be able to read a phrase like "closed convex subset of a Hilbert space" and effortlessly think to yourself "oh! there's a unique norm minimizer" -- if you have to piece that together from scratch every time, you're going to have a hard time -- reading papers and learning new fields becomes a dreadful slog, similar to how math in general becomes a slog for kids who don't memorize their times tables.