Sebass said:
I think because it makes your face look better: more pronounced jawline and cheekbones (at least that's what I've noticed).
Though the reason why most do it is probably because it's sort of a meme I guess. Like the 'V' fingers every guy used to do on every picture a few years back, even if they didn't even know what it ment. (It means 'Fuck you' in Britain)
Now I get to nerd at you
When the English army was over in France during one of the little scuffles we've had with them, the English longbowmen were causing the French some serious grief because they were just so awesomez. Hence, when longbowmen were captured by the French they cut off their middle and index fingers so that (just in case they escaped) they couldn't fire a bow again and would be ineffective for the rest of the war. The longbowmen caught onto this and started to waggle their two intact fingers at the French before they fired as a kind of taunt.
So basically it means "I have my fingers and now I'm going to shoot you with them, you foreign bastards".