Looking at various DIY's on incubators, the one difference I cannot understand is the open water placement for humidity.

In a plastic cooler open water is placed at the bottom or in metal trays with fans blowing heated air over the surface.

In a wine cooler or fridge sealed water bottles are used to retain heat and the fan is used to move warm air around the unit. No open water.

In both cases the egg boxes look to made the same. Why does the plastic cooler need open water? Thanks.