Can help if wanna keep cpu usage down (can drop heavily depending on fps throttle from 100% down to 20 for me) when unfocus window but stay in game as observer/player and gameplay continues.
Also when in replay or general game and have high speed like 2x or 20x it can go many more turns per second, when renderer is paused due to unfocused window. So try this option if u like to pause rendering in general options.
Bugfix for game, apply background pause option directly when changed in options.