I believe it is used to distribute the weight of live rock on a bare bottom tank. That way there aren't any "pressure points" from the rock. Not sure though. Could detritus get trapped under it? I guess it would have to be sealed down. hmmmm.........
I think that was used as a replacement, not to keep rocks directly on the glass but you could find a few other products reef safe that would perform the same concept. I'd imagine if you ran a bead of silicone, you cold basically seal the bottom off.
I was at the plastics supplier yesterday and he says theres 2 types one is UV protected and the other is not. The other difference is price as one cost 3 times as much. Is it neccassry to have the one thats UV protected.
Also why does it ned to be at least 1/4 shorter than the inside of the tank.
It helps to protect glass tank bottoms from falling rocks. Thats all I know. Im changing over to a new tank with a BB and hope to put it on the the bottom, but need to know if I need the UV stuff or not.