The practice originated from Egyptian craftsmen (at least that's as far back as I've seen it). Probably carried over into warfare from peasants who were used to doing it at work (or alternatively, were wearing their leather aprons to the battlefield...)
As for what it does, the metal studs will tend to stop a blade actually cutting open the leather. Not sure how effective it would be against someone trying to kill you, but when you're looking at an accidental slip of an axe, knife or similar it would cause the blade to bounce off, or at least stop it from slicing too far. You'd likely still suffer bruising (and in the case of an axe, broken bones), but you'd keep the limb, which at the time is pretty much the difference between a week or so off work, or being out of a trade altogether.
Not sure about historic examples. Leather rarely remains intact unless preserved in near ideal conditions, and of course hammering studs through it won't help. We do have bracers and the like, though I'm not aware of anything resembling actual armour.