I am a big believer in building character. I think that there is great virtue in forcing yourself to become accomplished at things. There are certain things in life - learning another language; learning how to cook or make pottery or fight; familiarizing yourself with canonical works of literature and philosophy, and so on - which are important to do not just because they are good and useful in their own right, but also because they teach you focus and self-discipline and concentration and mental or physical toughness and persistence and all other kinds of things which your grandfather probably deemed important.
It is good to be the type of person who can say: If I am set a difficult task, I can force myself to do it because I have done even more difficult things before, by choice, so fuck you.
How do RPGs feature in this? What does being a DM teach you? If you were a grandfather or grandmother, what would you say to an 11 year old as you sat there wagging your finger at him and explaining the virtues bestowed upon somebody who regularly DMs a campaign?
Discipline. You learn the value of spending time working hard on doing things which may not even have any sort of pay-off (many of the maps and other things you create will not end up being used at all), and you will often receive barely any thanks for your effort if at all.
Preparation. You will have to be ready for each session. You will have to have prepared and planned materials, of course, but you will also have to learn how to mentally prepare yourself to be on top of your game. This is more difficult than it sounds.
Thoughtfulness and consideration. You will have to think about other people - what they might enjoy, what they might not. You will have to take account of whether other people are enjoying themselves during each session.
Confidence. You will have to be in charge, the centre of attention, the one who steps into the breach, the one who guides and leads when guidance and leadership are necessary.
Decisiveness. You will have to make decisions. There will be many times during a game where you will be called upon to make a decision. You won't be satisfied with the options available to you, because you have been taken by surprise by events. But you will have to decide anyway.
RPGs are fun, but fun things can also improve you. How else does playing RPGs make people better?