This question already has an answer here:
- Why does 'Disarm' seem so trivial? 3 answers
The Disarm action is designed to knock an object out of the holder's hand and presumably into the holder's space. Because enemies aren't allowed to enter that space, nobody will be able to pick up the weapon in that time, especially in a duel. But picking the item back up is a free object interaction on the holder's next turn.
This means that, except for in corner cases, disarming an enemy doesn't cost them anything.
So what's it for?
Edit: This question has not already been answered, since the top answer of the linked question interprets the rules for Disarming very differently, which conflicts with my question.