Usually, you do this just for convenience and so you don't have to rely on any third-party software to maintain the OC. The other reason people modify the BIOS is to adjust the voltage settings for the card. I.E. if you buy a card that is a model down from the top end model and want to OC it to the speed of the top-end card, you will likely need to increase the voltage going to the chip to at least that of the top-end model.
Same principle as OCing a CPU really. You just don't have a BIOS screen for the video card that lets you easily change such settings as you would on a motherboard. All the settings are hardcoded into the BIOS image.