Gonna sound weird, but the ability to change a person's age at will, with the ability to make the progression/regression either physical, mental, or both at once.

Imagine you have a co-worker who acting like a little kid. You could turn them into what they're acting like physically so they can suffer the indignity of knowing they're not supposed to be so young and helpless. Or follow an age regression storyline where your cheating partner gets turned into a helpless baby that you now get to take care of and embarrass as they still retain their mental state.

Also, imagine you and a friend are going to some event like a movie or a fair or something else where kids get cheaper tickets. You or your friend could be shrunk down to kid size and then all you would need to do is find a place to change back once inside. Saving money!

The possibilities are endless. Also, blame the fact that a lot of cartoons from the 90s onwards that I watch/watched in reruns have some form of age regression stock episode where one or more of the characters goes through age changing shenanigans for implanting this idea in my head.