Your body goes into autophagy during a dry fast much faster than it does during a wet fast.

In autophagy, our bodies metabolizes lots of intercellular debris that is otherwise not good for us.

Also, your body ramps up the production of "endogenous water". Cells produce water as a product of metabolic reactions and producing more water helps to flush that same intercellular debris out of the body.

It's a little uncomfortable, at first, if you're used to "staying hydrated". Many of us conditioned ourselves to think we need to drink something, right around the time our bodies would start switching state. However, I got used to the new state very quickly (similar to how you stop feeling hungry, once you get used to going without food).