The BIOS is the first operating system a computer user sees. While you may run Windows or OS X, you also run a BIOS. The majority of the time, no one ever changes the BIOS no matter how many times the operating system may change. The LinuxBIOS project replaces the operating system on the BIOS chip with one of its own design. Why should you do this? For one, it loads Linux in about three seconds. This is actually a very important factor if you have to restart your Linux computer for any reason. Another nice factor is that it allows you to use some services, such as redirecting the console to the serial ports. Rack mounted servers usually have this in their BIOS settings, but a converted old PC will most likely not have this feature. So for no money you can squeeze more services and performance out of your old hardware.
Of course, make sure your hardware is supported before installing and have a copy of your old BIOS just in case you want to go back.