Not only a deeper understanding, but to retain the mental state of when you first didn't understand and be able to empathize that in your student's mind.
Some problem domains simply becomes so hard to teach, because once you learnt the domain, you tend to forget what made it confusing or hard, and thus unable to "teach" it to someone who hasn't understood it.
Some problem domains simply becomes so hard to teach, because once you learnt the domain, you tend to forget what made it confusing or hard, and thus unable to "teach" it to someone who hasn't understood it.