GMing Makes You a Better Person: Improvisor
You've brought your players to a dimly lit and a bit dingy of a tavern, only tended to by the single bartender who watches them enter. They start to give the bartender a hard time, what do you do?
If you're answer is "I... Uh... *fart noise*" then take more time to think about it. It's okay to not be quick. You can take the same approach the next time you're in a similar situation in your game.
Improvisation sharpens your wit.
It's a blade that you will hone as you use it again and again in your games. Until finally, you reach a point where you're not even prepping for the next one because you were busy as hell all week, but you can wing it and be totally fine.
Your NPC dialog will get better and better the more you go off the cuff. And to be honest, it's pretty much inevitable that you'll have to improv at some point unless you're insane at prepping.
You'll listen better.
Improv is not just about quick retorts but also understanding the current situation and factors effecting it. You'll become more attentive as a reflex and be present to keep the scene moving.
It improves your ability to actively listen, focus, understand subtext and better catch on to emotional cues both in and out of game.
I've noticed that both in and out of game my conversational skills have improved immensely as well as my vocabulary and I attribute a lot of that to being a GM.
The Dragon Potion
One hilarious example of great improvisation in recent memory is when I had a player carrying around a potion for about one IRL year. The potions were special in that they couldn't be identified but instead had pictures inside them made out of bubbles that loosely described what they did.
This potion in particular had animated images of a human turning into a dragon. So once we got a really tense moment in our story, just as the players were negotiating with the duke of a town, the player popped the cork and downed the potion in one swig, only then to realize that the potion made the PC think he was a dragon.
What ensued was his fantastic improv of his character acting out all of the character traits of a big, bossy and greedy dragon for the rest of the session.