The way I see it is that grain makes the film look more detailed than it really is, it can also hide compression artefacts and blurriness.

I don't know the psychovisuals behind that. Maybe it adds some high frequencies that compression often washes out, or maybe acts like some kind of dithering.

As for your eyes, I am pretty sure that they have grain, that's how quantum physics work, you just don't perceive it because your brain filters it out. But again, I don't know how it interacts with film grain.