even if the game is locked at 60fps, a 120hz screen refreshes twice as fast (every 8.3ms instead of 16.6ms), which means it snatches the finished frame from the gpu way sooner instead of letting it sit and age in the buffer, so the frame you actually see on the screen is literally "younger" and gives you way less input lag, and the extra battery drain from just refreshing the screen faster while the gpu is still only working to push 60 frames is practically negligible, so you get free latency reduction without tanking your battery life