To the right are engineering drawings of a .29 in long screw and a .237 in long screw. These are used to assemble parts of the Roomba by iRobot.
To the right is the button plate. This piece sits inside of the button ring and in the center hole of the face plate. The button plate provides protection for electronics closer to the exterior of the robot as well as provide a more aesthetically pleasing look.
The button ring serves, as far as I can observe, no other purpose than to be aesthetically pleasing. The button ring holds the button plate and sites inside of the face plate.
The face plate, shown to the right, provides protection to the robot as well as increases the aesthetics of the robot.