Grain tends to add a perceptual sharpness that gives the eye some fine edge detail which it thinks is added detail. This ends up making the depth of field fall off on film feel like it's wider DOF but its really the same.
On a digital camera, the low pass plays an important part in this also. If you put in a very weak or no low pass filter you'll get a hyper sharp image with a DOF that's falls off very fast and is perceptually a shallower DOF.
Gate weave/sway also has some added effects...



