"If you can't do, teach"

We've heard the phrase, "If you can't do, teach". I've often wondered the reasoning behind it, but recently a few reasons have surfaced to my thoughts:

1. We use teaching as a learning tool for ourselves. By attempting to teach other people, we discover the blind spots in our knowledge and can go back and teach ourselves these things to form a coherent picture. People who like this way of learning would naturally gravitate towards teaching. 

2. The "do-ers" don't even realise there's a problem/solution to be taught. They are of a certain skill level where the knowledge they already know allows them to unconsciously solve problems without them even realising a roadblock. The teachers find smaller things a larger obstacle for them. How many times have you seen a topic of education and thought, "what?! They're even teaching that? I thought everyone just knew that!!"