The primary function of a **motherboard** in a computer system is to act as the main **circuit board** that connects all components of the computer. It houses the CPU, memory, and other essential hardware, providing pathways for data communication between them. Additionally, the motherboard contains slots for **expansion cards**, ports for input/output devices, and connectors for power supply. This makes it crucial for the system's overall functionality, as it facilitates **communication** and coordination between all parts of the computer.