Variously: digital network design, calculus, C (beginner & advanced), discrete math, miscellaneous stuff-you-probably-should-know-as-a-person-who-programs-computers-sometimes.

Mastery helps because you can answer "why": why is something useful, why would you care to study something (sometimes those two have different answers!), why certain algorithms give the results they do. Anyone can learn, say, how to calculate a remainder; mastery helps you connect remainders to other related ideas (cycles, clocks, symmetries of geometric shapes, cryptography...).

Also, mastery helps you answer unanticipated questions and to come up with alternate explanations when your audience doesn't get what you're trying to say.

It doesn't help when you forget that you know something--it's easy to gloss over basic concepts that have ingrained themselves in your mind so much that you don't even notice they're there anymore.

This is not to say that I've mastered anything, or that 'mastery' is a final, well-defined state of existence...