Hills allow for the tradeoffs between units to shift and introduce another factor into calculating how to play a matchup. For example, artillery can become viable to mix into unit mixes much earlier if they're being sent to an area where hills provide cover from direct fire. It also makes it possible to attack pd that was placed poorly or keep things like radar safer.
It also makes any sort of large impassable terrain much easier to look at. I'm not even "the aesthetics guy" but I don't think there is very much more awkward in this game that having a straight 90 degree impassable slope rise out of entirely flat ground. It's obnoxious and immediately makes you think "this is a game" when you see it.