My hair seems to absorb more.
The logic around it (as I understand it) is that hair only has a certain amount of 'free' room that a molecule (whether water or conditioner) can enter. If your hair is wet, then water is taking up some of that space. However, if your hair is dry, the conditioner can get to EVERY exposed spot of your hair - thus making it a deeper/better treatment.
I've found, that sometimes it's useful to thin your conditioner a TINY bit (like 8 drops of water to a handful of conditioner) and that helps it 'spread' better (since they make it of a thickness assuming that it's going to mix with a LOT of water on your wet hair).
I think the reason they do it on wet hair in the salons is convience - they are applying it AFTER the shampoo, ya know? Doesn't mean it's better.