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I use it in the Mbuna tank with all the rockwork. Thankfully, my Mbuna don't dig that much.....but its a good feeling knowing the eggcrate is there. I don't think its something you have to have, but definitely lower my anxiety about all the rocks being in the tank.
Not only does it protect the bottom of the tank against shifting/falling rocks, it also disperses the weight of the rock so the load is more evenly spread across the tank's bottom. IMO, it's a really good idea whenever you're using a substantial amount of rock in a tank.