Your business material will look much better and reliable if it is organized, and folders are excellent tools of organization. The most usual folders are bi-folds, and they usually have one, two or more flaps to hold the pieces inside them. When these pieces are sheets of printed paper, they are called inserts.
Folders are great for practically any promotional use, but they also work as a protection for the pieces inside them. They keep your promotional material organized in the same place. Folders can be configured (by adding or removing inserts) to different clients according to their interests.