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I wrote BIOS for notebook computers for several years. The BIOS does a lot of things while the OS is running.
A major task is to inform the OS when many events happen so the OS can look smart (as if it somehow figured these things out on its own). For example, the BIOS tells the OS when: the power button is pressed, batteries are inserted or removed, AC power comes or goes, the system connects to or disconnects from a docking station, hard drives and or certain types of optical drives are inserted or removed.
Most portable computers have features that you can access/control through Fn keys and through OS-level applications provided by the manufacturers. The BIOS responds to these hotkeys and has code to interface with the OS-level apps. Features like controlling screen brightness (which certain OSes want to appear to control) or controlling bling LEDs fall into this category.
Perhaps the most important task of the BIOS is to shut down the system when the power button is held down for more than 4 seconds (to recover from OS hangs!).
The biggest benefit to having OS control over BIOS now is to control hardware level variables such as fan speed, temperature gauges, etc.