The purpose of the BIOS (Basic Input/Output System) in a computer is to perform essential boot functions and system hardware initialization when the computer is powered on. It provides a firmware interface between the operating system and the hardware, ensuring that all components are recognized and operational. The BIOS conducts a POST (Power-On Self-Test) to check system integrity, loading the bootloader to initialize the operating system. Additionally, it allows users to configure hardware settings and optimize system performance through its setup interface. Overall, the BIOS is crucial for a computer's startup process.