It brings awesomeness. As an example, the players are searching for the location of the MacGuffin and encounter a powerful devil. The DM expected it to be a fight, but instead one of the players decides to try and bargain with the devil, offering his soul in exchange for the location of the MacGuffin.
D&D has no rules for this, but it's well known in lore that devils and demons bargain for souls, offering power, information, wealth, etc. for those souls. The DM decides by fiat that such a bargain is possible in D&D and negotiations begin. Heck, the warlock class is based around these types of bargains.
Without DM fiat, the player gets turned down and a great opportunity for RP and a change in the direction of the story by the players is missed.