The BIOS is a reminder that your computer continues to pay tribute to the great IBM PC XT in the sky. Nowadays, though, BIOSes still have very useful functions; assuming your BIOS isn't stored in a separate partition on the hard drive. BIOSes also serve as useful reminders as to how recent your computer is; simply look at the copyright date to see how new your computer is. Example:
Award BIOS v7.00PG, An Energy Star Ally
(c)1984-2002, Award Software Inc.
The BIOS's Setup menu is one of the most important things in your computer; it is here where you can view legacy settings like 'shadow Video RAM' and 'Mem Hole 15-16M'. Generally, these menus' 'Help' files will be in broken English, reflecting the brilliant programming skills but poor language of the Taiwanese developer of your motherboard. The only times you'll ever have to deal with BIOS setup is when you've installed a new hard drive or to disable integrated onboard cruft.
A BIOS is not just limited to the motherboard, though. Video cards have their own BIOSes, and so do SCSI controller cards.
Generally speaking, the fastest way to destroy your computer is to try and flash a BIOS from a protected mode environment such as Win32. Assuming your BIOS doesn't have any write protection enabled, attempting to do this will have no immediate effect - until you reboot the computer, when the computer must enter real mode again. The proper way to do this is to use a bare-bones Win95/98 floppy disk created using FORMAT A: /S, rebooting, and running the executable zip archive from the floppy. ASUS's LiveUpdate was created to address this problem but the program can be a bit flaky at times, and of course it was created only for ASUS motherboards.