What is the point/job of medium tanks?

As a semi-new player that has only played heavies and frontal armor tank destroyers, I don’t see what mediums excel at, just looking at the stats of them they seem like heavies are just better. Any tips are appreciated, I’m just wanting to play a tank line that isn’t as slow as the E4 or 57 heavy, if it matters, I want something fairly nimble but not a light tank.