Judging by the way both Spike and Angel used it, I'd explain the bumpy fangy face as a combination of factors. As BotD said, I could definitely see it's usefulness as a visual tactic, to unnerve and scare (and it definitely has the desired effect) as well as in order to bite. The whole scaring idea makes sense as well, because Angelus refers to the blood tasting 'sweeter' when the victim is scared.
I also think it does have an effect on the strength, but not in the sense of making them stronger per se, more blood-lust driven, sort of like a burst of adrenaline. It's the game face I guess... when you put it on it puts you more into the headspace for a good ol' ass kicking session.