In theory, they increase humidity overall. But it could work either way. I have one snake that wraps himself around it, I guess to be close to the humidity. I've seen him sit in it once. But my other snake doesn't have any interest in it, and her tub's humidity is at about 50% all the time. (Give or take).

Mine is just a variation of the one with a door cut out. To me, since the humidity is going to evaporate upwards, it makes more sense to have the hole in the top. Not to mention, that way the water doesn't leak out. I only have holes in the sides of my tubs so when the water evaporates up, it hits the top and collects. Humidity then goes up.

(I feel so scientific... )