DIY Incubator - exposed water or not ???

[JohnNJ]

Depends on whether you want to keep the humidity inside the incubator up, or if you control humidity inside your egg boxes instead. If the egg boxes are tightly sealed, you can do that, otherwise they tend to dry quickly if incubator humidity is low.

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?