Why would a robot or computer be used for car manufacturing?

An example of car manufacturing using robots is in the paint spraying. A human sprays one car. The process is tracked by computer. Then. Cars move along the "belt". A computer stops a car in a zoned area and then activates the robotic spray arm. On a line might be 4 plus vehicles all being sprayed at the same time. Computers are also used to program the EMS (engine management system)