Resolve potential compatibility problems with mainboards and chipsets, and the rare occasion very mild performance enhancements in specific conditions due to slight timing changes in the core or memory controller setting alterations.
Voltage adjustments to the GPU/memory to increase stability and/or improve reliability at high AGP bus speeds, or do decrease power consumption.
Any number of possible reasons, only a few of which I've listed above.
I NEVER recommend anyone update the graphics card BIOS unless they have an substantial compatibility issue and all other potential resolutions have been exhausted with no remedy found.
Updating a graphics card BIOS is considerably more risky and problematic then a motherboard BIOS, I've seen countless graphics cards die after an attempted BIOS flash.
If you do choose to flash the graphics card BIOS try to ensure that it is NOT the bootable graphics card when you flash it if at all possible.
Of course the ideal solution would be to let the manufacturer to do update themselves, but that's seldom a viable option.