Why a Tripod Plate Comes in Different Shapes
On a tripod designed to support a camera, the tripod plate is the important link which connects the body of the camera to the tripod. It is designed to prevent slipping of the plate, to allow the user to hold onto the camera without dropping anything. As tripods are often manufactured separately from the devices they are designed to carry (and may not even be produced by the same manufacturer), it makes sense that the plate should come in a range of sizes in order to best fit the shape of the camera's body.
The different shapes of the tripod plate have their origins in different manufacturing styles. There is the Bogen quick release system, which holds the plate in place on the tripod, but it is known for only having One Size for all different types of plate. Most cameras are in fact designed to have plates which fit into the Arca-Swiss system, which is held in place by jaws.
While there are generic makes of these plates, most digital cameras will have specially fitted plates, which help to keep the body of the camera secure. Some plates also have a special anti-twist design which prevents the camera from rotating, which can damage it.