It's used for the master shot. When you shoot, you always shoot a master, which is the entire scene with a wide angle. Then, you go in for the closer shots. This gives the editor something to work with. He has an entire scene to start cutting into, instead of having nothing to start with (and it guarantees there won't be something missing). It's also used for, well, when you want to use a wide angle to show something. It's up to you! A DP is an artist, the camera/lens is his brush, the film/video is his canvas.