The objection “Allah already knows every deed, so the balance is useless” overlooks the distinction between acquiring knowledge and manifesting justice. The weighing is not a device by which Allah learns new facts.
It may serve to display accountability, recompense, and justice to creatures. Yet it does not follow that every divine act must have a specific humanly discoverable purpose imposed upon Allah.
Claiming an obligatory external purpose differs from acknowledging possible wisdom. Understanding a wisdom is helpful, but failure to comprehend every wisdom does not invalidate the revealed event.