I think I read somewhere that extending battery live is Valve's main priority for next Steam Deck
With a 120Hz screen, you can run the UI and overlays at 120FPS, run cinematics at 24FPS or 30FPS and the game at 40FPS or 60FPS. All fits neatly into 120Hz.
How does a 99% static UI or overlay benefit from 120 fps?
They have it a lot harder than something like Switch.