Is grain actually an aesthetic boon to digital art, or does it seem that way just because we associate it with printed work, and we associate printed work with higher quality.
Grain increases perceived sharpness and decreases gradient banding, so there's more benefits than just nostalgia.
Grain in photography can evoke feelings of roughness and emotion. It's especially effective in portrait photography. If there are subtle gradients, grain also hides those and prevents the obvious look of posterization.
Those might be the same thing. How a thing seems and its aesthetic quality.