The main reason to upgrade your BIOS is to allow your PC to talk to new peripheral devices that have been introduced after your PC was built. Some devices will not work with an old BIOS.
Plug and play or PnP support is another reason to upgrade your BIOS. This allows your PC to automatically configure itself to recognize new expansion cards as you install them. However, your new expansion cards must also support PnP. New hardware installations can be much easier with PnP support.
It is very rare, but your current BIOS could have a bug. No, not an insect, but a problem with the BIOS program code. Most manufacturers fix the bug and release a newer version of the BIOS.
Finally, you may want to upgrade your BIOS to get new features. Virus protection and password protection - to name a few.