Generally for the sake of the illusion of 3 Dimensions and perspective. There are plenty of artists beside Escher that represent multidimensional space, but he does a particularly good job of it. The last stereogram at the bottom is the thinker. I ran out of characters. (you can always go to the site.) Stereograms can be annoying sometimes when you can't see what it is, and everyone else can. It can be like that, then suddenly one day a picture can jump out. It can help if you know what you're looking for. One problem with these may be looking too hard, when the eyes are probably better off relaxing.

very nice clip!