The BIOS was an abstraction layer. In the old days, not everything was 100% IBM PC compatible. There were lots of weird graphics cards. Some systems had incompatible disk and keyboard controllers.

There was no memory protection in Real Mode, so you could always poke the hardware yourself, but something written on a Tandy wasn't going to work on a Zenith unless you supported both, or ran everything through the BIOS.

Over time, the OS took over the HAL role, with the BIOS only being used until the OS could load native drivers. Now it's UEFI... same idea with a higher greater level of abstraction and modularity.