Its in-game description reads "Absorbs power from infinite reaches of time and space." This brassard is a penultimate weapon : so its stats are quite high. Even if its got 5 elementals stats at 0, the fact that Monica can charge her magic projectiles can compensate this. The Mobius Bangle's high stats can easily defeat enemies. And as a penultimate weapon, its Attack value will go up faster (+3 instead of +2), so its power will increase faster than non-penultimate weapons. You can perhaps find the Mobius Bangle in dungeons' chests, howewer it cannot be invented or bought.