They help in usability testing, too.
Prototypes can be simple or complex.
Sketches of dialog boxes, windows and menus are the simplest form of prototype. Pictures represent major topics of the interface, and can represent the flow through the product. All you need is a pad of paper, a pencil and rudimentary drawing skills. One benefit of this technique is that it does not require any computers or computer skills, which means it can be done by anyone and in group settings. Group prototyping is a great way to get team members and users involved and incorporate their ideas.