Grain in film was silver halide chemistry; grain in digital work is a choice. A quiet layer of per-frame noise breaks up the sterile smoothness of rendered gradients, keeps dark regions alive, and signals cinema rather than screensaver.
It also does technical work: like dithering, grain masks banding in gradients by trading it for texture the eye forgives. The rule of thumb is that grain should be felt, not seen: visible at pause, invisible in motion.
Photism ships grain in its post stack for both reasons, the texture and the banding, tuned to stay under perception at performance brightness.