Limiting depth of field is a very useful way to emphasize specific elements of a photograph and for that reason it's almost always used for portraits.

It also compensates for portrait photos being best taken from a distance using a telephoto lens. The reason those are best is because it captures the face people remember instead of the face they actually see. The reason compensation is needed is because the same lens configuration will get a much smaller depth-of-field up close but a much higher one at a distance.