DIY Incubator - exposed water or not ???

[Domepiece]

I understand what you are saying but in all of the plans I saw marine/beverage/food type plastic coolers had open water trays or water on the bottom. None of the fridge types had that.

Why is keeping the humidity up in the incubator important in a cooler and not in a fridge?

Its important in both. but not neccessary in either if they are keeping the humidity up in just the egg boxes alone. You may have just looked at ones that just happened to have one or the other. Some people put open water in both just to make sure the humidity is up to par to be cautionary. I just built one out of a cooler and am just using water bottles to keep the temps stable and so the temps can recover quickly after opening. Im doing this because I intend to keep my humidity up within the egg tubs themselves therefore it isnt necessary to have open water within the cooler itself. Hope this answers your question. GoodLuck.