John Calvin Commentary Numbers 31:51

"And Moses and Eleazar the priest took the gold of them, even all wrought jewels." — Numbers 31:51 (ASV)

And Moses and Eleazar the priest took the gold. It was fitting that this should be added, so that no one would suppose that Eleazar made a profit from the generosity of others. Moses, therefore, relates that whatever gold was offered was faithfully set aside as an ornament for the sanctuary. When it is said, for a memorial for the children of Israel, it can be understood either actively or passively; namely, either that the gift could be a monument of their gratitude, or that it might win favor for the people in God’s sight, as if that offering of expiation was brought before God, and represented all those who thus professed that they were preserved by His grace. I prefer the latter sense myself, that is, that this memorial was set before His eyes, so that God might continue to be favorable to His people.