Every cat I have owned has loved to sleep in cardboard boxes of any size. It could be a plain box, a sheet of cardboard, an old cookie box... Why do they do this? Is there some smell they like? Is there a chemical product in cardboard that attracts them?
This has to do with security. A box has 4 sides so they feel safe and protected since they are closed off. It also gives them the element of surprise and if anything happens they can easily escape/attack etc. (src)
They like the security of an enclosed space. A box resembles a cave and may make your cat feel secure knowing they are protected and, theoretically, cannot be attacked from behind.
Boxes are fun! Lots of cats seem especially playful in and around boxes. Cats enjoy jumping in and out of a box, pouncing, stalking and climbing atop boxes. Cats are little predators, and a box is a great spot from which to pounce on their prey, even if the prey is just a catnip mouse toy.
Boxes are cozy. Try putting a soft blanket in the bottom of your cat’s favorite box. The sides help retain the cat’s body heat making a comfy nest. (src)