Better level of learning
I used to think that if I could remember everything I learn, I would be successful.
I was completely wrong.
There are different levels of learning.
In teaching, most of what teachers share isn't based on memory but on progressive understanding.
When you have to verbally explain the concept, you're giving it more color. The more you do it the better you fill in your knowledge gap.
To learn better, one cannot help but must teach it back to an audience.