The folds that make up an airplane give it the shape it needs, control the distribution of mass, and add stiffness where needed.

By itself, a sheet of paper is not very stiff.

Hold a sheet of paper by the edge and notice how it droops limply downward. Hold your multifold airplane at the rear center, however, and notice that it doesn't droop.

Paper, because it has the ability to hold a fold, can be made rigid.

A fold that increases the height of the body of the plane will make a plane that resists drooping under gravity. Small folds at the back edge of the wing or the body allow you to control the flight of the plane.