The answer isn't straight forward. Some users elect to use a certain BIOS for speed or stability gains they might have on a particular game for certain game-play sections. It highly depends on a case-by-case setup. What endrift says isn't wrong. You generally won't see a benefit using an external BIOS vs the mGBA internal BIOS, on average/most of the time. The option to use BIOS are for those who codes their own / borrow a special built BIOS, trying to sequeeze a few extra FPS and what not.