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A Knuckle Duster seems to be strictly worse1 than Powerful Fist, still it is a Monk weapon.
Why would a Monk use it?
I am insterested only in mechanical answers, as you can explain any mechanically bad decisions with role-play.
1 ) 1d4 vs 1d6 damage, no Finesse
Monks get "cold iron/silver" unarmed attacks at level 9 and "adamantine" unarmed attacks at level 17. If you know you will be fighting a creature before those points (as early as level 2-ish for cold iron/silver and 11-ish for adamantine) that you would benefit greatly from having such an attack available, it may be worth having a set of knuckle dusters around to address that need. They appear, currently, to be the only reasonably metal-made bludgeoning Monk weapon (except possibly the pantograph gauntlet, from the descriptive text) and the only common metal-made Monk weapon.
For that matter, breaking stuff could be a valid reason to keep standard-grade adamantine knuckle dusters around from level 11-ish. It's unclear if the level 17 class feature includes the full benefits of adamantine including reducing target objects' Hardness (the text is "Your unarmed attacks are treated as adamantine.").
If you have selected non-monk features that require using weapons, knuckle dusters suit the need. For one example, Double Slice from the Dual-Weapon Warrior or Fighter archetypes, doesn't allow for use with one or more unarmed attacks ("Requirements You are wielding two melee weapons, each in a different hand."). This may be a stop-gap while you wait to take Monastic Weaponry and find the time to go shopping for more beneficial weaponry.
It can also be the target of some magical effects, such as magic weapon (which is more generally useful than magic fang and not restricted to Primal) and disrupting weapons, that don't allow for selecting unarmed attacks.
These points assume you did not invest in Monastic Weaponry because there are better Monk weapons with that Feat. Knuckle dusters have the benefit of being simple and common.
Overall I think it is unlikely that you would prefer the knuckle dusters, but there may be edge cases where you would have a reason to do so.
One reason is that this being an actual weapon, and you might find a magical one that deals extra damage — a +2 knuckle duster for example would deal better damage if your monk is strength based.
Even if you can transfer runes from a weapon to the handwraps, early on you are not of high enough level to do so for a +2 rune, so if your circumstances were that you have such knuckle dusters, you could not transfer them and would be better off using them.
P.S. I originally answered before the optimization tag was added. In that context, you might get some knuckle dusters with your name or another message spelled out on them as ridges in mirror image, so as to leave imprints of them on the victims of your attacks. There is historic precedent for this with maces, for example the Narmer macehead of ancient Egypt. While you might cause less damage, it could help you with getting known (and feared) among your enemies.
