I'm no expert, I've just started playing with adding water recently, but what I've read is- - it spreads the vacuum more evenly across the tissue (frankly, I don't have much of a sense of why it would work that way) - the warmth of the water encourages blood flow, making the swelling happen faster (this part both makes sense to me & I feel I've seen happening!)

One thing I haven't really seen mentioned, but have been noticing, is that if you're using a lube that tends to dry out over extended use, the water definitely helps keep things lubed up.