Why corrugated card has a wave inside it
The strength is not in the paper, it is in the geometry.
Pull apart a piece of cardboard and you find three layers: a flat sheet, a wavy sheet, and another flat sheet, glued together. The wave is the whole trick. On its own, paper is floppy in every direction. Bend the paper into a series of arches and glue the arches between two flat faces, and you have created a structure where the flat faces resist being pulled apart and pushed together, while the arches hold them at a fixed distance and stop them collapsing towards each other. It is the same principle as a steel beam with a web between two flanges, executed in a material you can tear with your fingers.
This is why cardboard has a grain. Stand a sheet so the waves run vertically and it will carry a surprising load, because you are pressing straight down the arches into their strongest axis. Lay the same sheet flat and press, and it crumples immediately, because now you are asking the arches to fold sideways, which they are delighted to do. Every box is built with the waves running top to bottom for exactly this reason. The panel walls are not walls, they are a set of tiny columns standing shoulder to shoulder.
The wave also comes in different sizes, from coarse and deep to fine and shallow. A deeper wave means more distance between the faces, which means more resistance to bending and more cushioning against a knock. A finer wave means a smoother printing surface and a crisper fold, which matters when the box is also the shopfront. Choosing between them is a genuine trade off, and the choice is usually made months before the box ever holds anything, by someone balancing appearance against how many times the box will be dropped.