The purpose of a **CPU**, or **Central Processing Unit**, in a computer system is to serve as the **primary processor**, executing instructions from computer programs. It performs basic arithmetic, logic, control, and input/output operations specified by the instructions, functioning as the brain of the computer. The CPU's efficiency and speed significantly affect the overall system performance, determining how fast processes are completed and how many tasks can be managed simultaneously. Essentially, the CPU is crucial for executing and managing tasks within the system.