Art on canvas generally gets stretched before framing. Stretching involves tautly pulling the canvas around a wooden frame and stapling it all the way around. The art is then ready for framing. Gallery wrap is used when the art is to be floated (discussed below) or hung without a frame. Gallery wrap involves using thick stretcher bars and wrapping the canvas around the back, where it is stapled (with regular stretching the staples are on the side, but it does not show due to the frame).
The heavy bars and the crisp sides make for a finished presentation, however, in order to do gallery wrapping there needs to be enough extra canvas. Sometimes the art is created in such a way to allow the image to be wrapped around the sides. Some art lends itself to this presentation, and others require a frame.