I'm a newbie. Not done any BIOS modding myself yet, but I think that by modifying the BIOS you can change memory timings. You can't do that through MSI Afterburner for example.

An advantage of BIOS modding is also that the modification is on the card itself. You don't have to re-apply them on each boot or if you use your card on another rig.

There's disadvantages as well. BIOS modding must be done very carefully as you may brick your card. And you probably can forget about warranty if you tamper with the BIOS.

Anyway, it all comes down to how much performance you want to get out of your cards and how much hassle you want to go through.

I'm definitely going to try BIOS modding my cards (Sapphire RX580's), but first I'll see how far I can get with Afterburner or similar.