Lens Controls for Creating Images
Focal length determines area coverage. The shorter the focal length is, the wider the area that is covered by the lens. Wide angle lenses, thus, can be used to cover a larger area without having to move the camera; telephotos allow you to enlarge distant details.
Changing lenses is frequently more convenient than moving the camera, especially if it is mounted on a tripod. Often it is the only possibility; for example, the size of a room may prevent you from moving farther away.
While area coverage is a good reason for using different focal length lenses, lenses can do much more than just cover larger or smaller areas. They can be major tools for creating effective, beautiful, unusual, ...