Noise increases the perception of detail in a particular photo.

Both are converted from a digital color image to a black and white image. One has noise added to it to simulate film grain, one is clean, as is converted from the original image.

Noise helps trick the eye into thinking there is detail in areas which seem very “clean”, such as skin, walls, and fabric. We typically perceive very fine patterns in these objects which might not be apparent in photos taken digitally, and so adding noise can serve to “fake” the texture of these objects.

Cleanly converted

Added noise