I seen people post where they get egg crates from but never what they use them for on other forums that I am not registered on so couldn't ask them. What are the egg crates used for with crested's?

Climbing and hiding spots.Makes it alot easier to just throw them away when they become soiled.Then to clean everything off.

New you would have the answer Jeremy!

Do they use it as a substrate lying flat on the bottom or lean it against the side as a wall?

Do they use it as a substrate lying flat on the bottom or lean it against the side as a wall?

No not as a substrate because they would weasle there way underneath and just crap on the cage floor anyways.
You can stack multiples on top of each other or lean them on the sides.

Thank you