To deceive the eye
There are pictures of Trompe L'Oeil in what, to me, is an unusual situation at Clearly, More Than Meets The Eye. I suspect the fish tank is the best as it is photographed from the side, but without being there in person it.
Years ago I saw photos of a bathroom done with Trompe L'Oeil, with some tiles broken to add to the illusion of it being outside within a roman villa, something that would suit my bathroom, and something that I will seriously consider when I get around to renovating my bathroom. Unfortunately I can't find something similar to it on the web.