When rats wash a lot there's a sort of a sweetish wet fur smell. I like it, and it brings up all kinds of associations with late night hacking, or gaming, or whatever else, since they hang out on my lap all night grooming themselves.
This is why rats are pretty much the ideal hacking pet. A dog smells bad and usually doesn't fit on your lap. A cat may or may not deign to sit on your lap, and may or may not decide to claw you if it's feeling particularly content, but will certainly be shedding hair into your face and decide the keyboard is a better place to relax anyway.
Rats hang out and do their thing while you hang out and do your thing.