Why is it nice? Is it that you're used to seeing it and its absence feels wrong? Wouldn't you become accustomed to its absence with exposure? I've found this to be the case for me.

Maybe it's like photography. I can edit a photo of a landscape to look how it looks in person, or I can edit it to look, to me, how it feels to see it in person, or edit it for some other feeling, of course.

Adding noise improves the perceived detail, which is important at low bitrates.

Well, to achieve a low bitrate, you have to fake the noise. That's the whole point of the submission.

It gives a movie texture. Compare Lord of the Rings vs. The Hobbit. Even bad (for todays standard) CGI fits perfectly vs. the clean fake looking one.

"nice" is not a rational argument, it is a qualitative/subjective/irrational one

This is why I, for one, only watch movies that have no subjective quality!

Fight Club without the grain is irrational