The primary function of a motherboard in a personal computer is to act as the main circuit board that connects various hardware components. It houses the CPU, memory, storage, and peripheral devices, allowing communication between them through data buses. The motherboard also provides power distribution and connectivity options such as USB, Ethernet, and audio ports. By integrating all essential components, the motherboard plays a crucial role in the overall functionality and performance of the computer system, ensuring that all parts work harmoniously.