Eggcrate under sand?

[Smotz]

What is the goal of eggcrate under the sand? I read it is supposed to stabilize the rock - how does it do that?

 

[BluewaterLa]

It may stabilize the rock giving the uneven surface of reef rock better area to contact with the square mesh rather than the smooth glass.
It also protects the glass from falling rocks that may hit hard enough to crack the glass.
Another thing I hear is that it distributes the weight of pressure points of rocks on the glass to a broader area.