Obviously not every game is doing that level of styling, but often a lot of stuff might still be happening when a game is "paused" for various reasons e.g. aesthetics, or handling input for pause menus, which usually offer you the ability to change all the games settings, e.g. video resolution, volume levels, etc. and also provide you with audio/visual feedback when you make choices, which requires keeping the audio engine running as well as rendering to the screen @ 60 fps.
Taken from this comment: https://news.ycombinator.com/item?id=47825155
Obviously not every game is doing that level of styling, but often a lot of stuff might still be happening when a game is "paused" for various reasons e.g. aesthetics, or handling input for pause menus, which usually offer you the ability to change all the games settings, e.g. video resolution, volume levels, etc. and also provide you with audio/visual feedback when you make choices, which requires keeping the audio engine running as well as rendering to the screen @ 60 fps.