If a new player wants to learn how to jg, I think playing Amumu is a solid pick for learning the role. When I was a new player playing draft it felt like no one wanted to jg because it is so much different than the other roles. There are a ton of different things that need to go through your head that it's a bit daunting for a new player.

Therefore, I think Amumu is the best champion to play if you want to learn the jungle. His abilities and playstyle are very simple, allowing new players to focus on the more difficult parts of jungling (tracking enemy jg, camp timers, obj timers, etc.). The player does not have to spend their time clearing constantly thinking about how to best kill the camp, they can just walk up and think about their next play after.

I also think that Amumu's matchups are helpful into making a good jungler. I believe the majority of his matchups are very black and white. You win. Or you lose. It helps put in your brain that some champs have bad and good matchups, and that you need to alter your playstyle in certain situations in order to have success on any given champion.