Many people use egg crate as a false bottom, but some people also use it as a background for the tank. If used as a background it can be siliconed to the glass and used as a backer for the great stuff so that it doesn’t peel away from the glass as easy. People have also used the egg crate to create the background out of the tank and then silicone it in later as an easy removable background.
Both examples do have a wavy background appearance, but that could just as easily be carved into something else prior to finishing it. I think that having the egg crate behind the great stuff would allow many nooks and crannies if you carved the great stuff which plants would love to get their footing.
Have you ever created a background using eggcrate? How did it turn out?