If you’ve heard about egg crate but don’t know what it is, it’s simply louvered light diffuser. It can be purchased at your big box DIY stores, among other places, and typically comes in sheets that are 24″ x 48″. Plaskolite is one brand, but there may be others. Aquarists use it to line the bottom of their aquariums, typically under the substrate, but the reason is a bit misleading. You’ll read that the purpose of the egg crate is to distribute the weight of rocks in the tank. That is true but it’s not the only purpose. The glass floor or your tank is perfectly capable of supporting much more weight than you think without using egg crate. A more applicable reason for using it is that it can protect your tank from a rock fall that might otherwise impact the glass directly at just the right (or wrong as it may be) angle to exploit an already existing (but unseen) concentrated area of flaws. Glass flexes but it has a flex threshold at which point the integrity of the bonds are compromised. This is true with or without egg crate. But the egg crate can lessen the flexing and thus protect those weak points (existing flaws). Remember, not everyone uses substrate in their tanks.
Having said all of that, many species of cichlids dig in the substrate or otherwise rearrange the substrate to suit them. When they do this, they can’t cognitively consider the inherent danger from any instability that they might create. If you keep mbuna from Lake Malawi, they like to dig…a lot, especially if you have lots of rock cover. By utilizing egg crate between your rock and your glass tank floor, you eliminate the single strike point if a rock structure gets undermined by your cichlids. If rocks fall, they strike the egg crate, which prevents that single point of contact from a pointed or jagged rock. Furthermore, egg crate provides a more stable platform on which to build rock structures, even if it’s buried by the substrate.
I use sand substrates, and my cichlids dig, both mbuna and some Tanganyikans. It’s not uncommon to find several small areas of exposed egg crate under the rocks when I move them to clean. You might not like the look of exposed egg crate in your tank, but if you utilize lots of rock structure, using it is good insurance.