My understanding is that you should use Extra to increase the size of your toolbox, but still use them only if they make sense. For example, I recently learned about the different kinds of maps in C++ while going through a part of the codebase that uses them heavily. I didn't use that knowledge directly though. Maybe one day it will come up, maybe it won't. When modifying old code, I try to have a good reason when changing the way things are done. If I'm doing something fancy, I usually try to confirm it with a collegue first.

Edit: another good example of Extra would be to build useful tooling for your codebase. It's usually higher-level work.