The use of greenscreen and bluescreen is a common practice in film making nowadays to create environments or situations that are not possible to do safely with actors or equipment. It makes possible every cockpit shot in Star Wars or 90% of Avatar. Most of the VFX work I do involves keying of some sort.
Some shots need elements added onto existing plates, like adding more snow, a sky replacement, or adding a helicopter to a scene.
Layering Multiple Elements
Often, it’s necessary to layer multiple elements together to create a desired scene. Shooting elements separately gives more control over subtle nuances, and allows the composite to come together easier.