That is something you and the other disagree on. If you approach the subject matter as if you are trying to teach it to someone else, then your approach will be more thoughtful and more thorough than if you just want to learn it. Basically, it's a way to trick yourself into being more thoughtful in your learning.

If you have loads of self-control and dedication, by all means write yourself notes in vim and study them. For the rest of us mere mortals, it can help for learning if you are pretending to do something else (i.e. "teaching").