Most “film grain” is just additive noise akin to digital camera noise. I’ve modded a bunch of games for HDR (RenoDX creator) and I strip it from almost every game because it’s unbearable. I have a custom film grain that mimic real film and at low levels it’s imperceptible and acts as a dithering tool to improve gradients (remove banding). For some games that emulate a film look sometimes the (proper) film grain lends to the the look.