Today has been about arm(vambraces) and leg(greaves) armour. The style we've been working on, the big ring greaves, use metal rings as the decoration and signify that the leather is reinforced. Like some other things we make they date back to the early days when there was a dearth of specific things to make the armour look heavier and more protective. Metal rings were available in hardware shops although I've no idea what else you'd use them for. They're fastened onto the leather with small leather strips, we use individual ones but I've also seen some wonderful woven armour with the rings woven into the pattern.
They continue to be popular today probably because it still has a heavy and brutal look and there's no question about the leather being reinforced.