I'm bit puzzled, isn't VRR more for low powered hardware to consume less battery (handhelds like steam deck)? How does it fit hardware that is constantly connected to power?

(I assume VRR = Variable Refresh Rate)

Variable refresh rate is nice when your refresh rate doesn't match your output. Especially when you're getting into higher refresh rates. So if your display is running at 120hz, but you're only outputting 100hz: you cannot fit 100 frames evenly into 120 frames. 1/6 of your frames will have to be repeats of other frames, and in an inconsistent manner. Usually called judder.

Most TVs will not let you set the refresh rate to 100hz. Even if my computer could run a game at 100hz, without VRR, my choices are either lots of judder, or lowering it to 60hz. That's a wide range of possible refresh rates you're missing out on.

V-Sync and console games will do this too at 60hz. If you can't reach 60hz, cap the game at 30hz to prevent judder that would come from anything in between 31-59. The Steam Deck actually does not support VRR. Instead the actual display driver does support anything from 40-60hz.

This is also sometimes an issue with movies filmed at 24hz on 60hz displays too: https://www.rtings.com/tv/tests/motion/24p

VRR is necessary to avoid tearing or FPS caps (V-sync) when your hardware cannot stably output FPS count matching the screen refresh rate.

It reduces screen tearing without adding all the latency that vsync introduces.

Are there games running at 4k 120hz?

Call of Duty and Battlefield both run at 4K@120 with dynamic resolution scaling, PSSR or FSR.

Most single player games (Spider-Man, God of War, Assassin's Creed etc) will allow a balanced graphics/performance which does 40 in a 120hz refresh.

Full 4k - very few, but lots are running adaptive resolutions at > 2k and at 120hz

Touryst renders the game at 4K120 or 8k60. In the latter case, the image is subsampled to 4K output.