light diffuser on bottom of tank?

[trainfever]

I know a lot of people use the light diffuser with the 1/2" grids as tank covers to keep fish from jumping out. But recently I saw pics of a tank with the same grid on the botom of a tank which was going to be covered with sand. What is the reason for putting the grid on the bottom of the tank? I saw this and thought this might be a good idea to keep my engineer gobies from digging down the the glass bottom.

 

[wangotango]

To provide more of a cushion between the rocks and the glass is the main reason (incase something slips it doesn't hit the glass).
If you put the rock on the glass as opposed to on top of the sand, then you should have to worry as much about a burrowing fish causing the rock to fall over.

 

[geoj]

You don’t need the egg crate but some people want to protect the bottom of the tank from the live rock. The egg crate will spread the presser points out giving some added security. And if you happened to drop a rock it may be the difference between cracking the glass and not.