The base is your robots skelton and it main functions are to hold all the other parts together and to protect delicate parts from harm. A base can be as simple as a scrap piece of wood or as complicated as a space ship.
In many cases the design of the base is completely intertwinded with the design of the motion systems. Sometimes the mechanical components for a robot can be "borrowed" from a toy or other hobby. Radio controlled planes, cars and boats (including submarines) have been used to provide the base and motion system of robots.