The small extra bump is an alignment or anti-rotation pin. It fits into a matching hole, recess, or ridge on the bottom of some cameras—especially many video cameras—to help stop the camera twisting on the plate during panning or general use. It can also help the camera return to the same position when remounted on a quick-release plate.

On some plates the pin is spring-loaded so it can adapt to different camera bases. If your camera body doesn’t have a matching recess, the tripod can still usually be used normally with just the main mounting screw doing the work. So its purpose is mainly to improve alignment and reduce unwanted rotation, not to provide a second required attachment point.