Painting on deep gallery-wrapped canvases gives me the opportunity to continue the painting on the edges of the canvas. One obvious benefit is that it cuts the framing cost to zero. Another benefit, which viewers have sustantiated, is that it opens the painting and allows the eye to continue beyond the surface of the painting, turning the corners, so to speak. For me, it is also fun to see how part of the image wraps around the canvas, giving a clue to the surface imagery when a only a small part of the painting is viewed from the side.
Since my painting style usually includes textured paint and mixed media, any corner seams are well covered, creating an uninterrupted transition from surface to the four sides of the canvas edge.