When you make your opponent say "I quit" that's the most definitive ending to a fight you could possible have. Even after being KOd may fighters have wanted to continue after regaining consciousness but the referees don;t allow them to continue. They're depending on a 3rd person to decide the fight should end. When a fighter has to say "I quit" that is a 100% definitive ending to a fight. It doesn't rely on a referee's judgement or that of the judges. It's the only finish in The UFC that is entirely definitive.