The primary purpose of a motherboard in a computer is to serve as the central hub that connects all components of the system. It provides the necessary circuitry for communication between the CPU, memory, storage devices, and peripheral devices. The motherboard also houses slots for expansion cards and connectors for power supply and input/output ports. Essentially, it facilitates coordination and communication among various parts of the computer, enabling them to work together efficiently and effectively, making it a critical component for the overall operation of a computer system.