Bios, basic input output system does manage the hardware in a computer. if that is the
case, why do we need an operating system?

BIOS (Basic Input Output System) is not considered as the main functioning  system in the operating system.It partially handles only output and input operations only which is for the basic level. It can also handle the managing of the boot up process and other such low level processes.

Operating system is a major system for working of the whole functioning in the computer system that can process the execution of heavy process, large data addressing and other main functions.So, it is required for the major execution of task in the OS which cannot be done by BIOS