There's always 4 reasons in my mind for a BIOS MOD overclock.
1. Most importantly for myself, it's permanent. Once you establish the Mem/core clocks fan speeds and voltages that work best, you can lock it in permanently & thus keep these settings from one build to another. there's no need for helper apps to load up at boot time and slow your PC down. Also, you don't lose your settings if you have a crash or corrupted windows install for whatever reasion. S#1t happens and you never know when things might get ugly.
2. nVidia and ATI are notorious for NOT running the fans on their cards high enough to keep their cards cool, even at stock clocks and voltages. This is because they want to sell cards to the small HTPC enthusiasts who are more concearned about quiet operation instead of blazing 3D performance. This is especially true in MATX or SFF cases where evrything is cramped and poorly cooled. A BIOS mod can change the fan speed profiles to suit an overclock better and you can, thru trial and error, find a compromise between noise levels and cooling to suits your overclocking needs best.
3. Most cards can increase voltages thru a BIOS mod and therefore increase the OC capability in general without any physical mod to the card itself.
4. If your are having problems with the card, you can even blind flash the card back to its original BIOS, using another PCI video card, provided you saved the original BIOS in case anything goes wrong. There are plenty of 'Blind Flash' guides on the net to help you thru the ordeal and I've done it myself once, and it was easier than I thought it would be.
Of course, if you are afraid of voiding your warranty, then overclocking is not for you. Any overcklocking inherently comes with risk and not for the clumsy, un-knolwedgable or faint of heart.
I've lost my fair share of hardware, but saved thousands of dollars in the end over buying the more expensive top-teir 'uber' parts. Most times, the middle teir parts outperformed the stock-clocked top-teir parts. =)