[dtashima]

I think it's partly motivational. There is definitely a level of understanding where it feels like you understand a concept, and then when you try to teach it to someone else, you realize when they ask basic questions that you don't understand it as deeply as you thought you did.

So, if you think you understand something, it's worth trying to explain it simply, so you can push your own understanding of the subject.

[sh87]

I think this is closest to what Feynman (I think) wanted to convey.

[jshelly]

I like this idea