When the women brought their vanity mirrors as gifts to the mishkan, Rashi (Shmos 38:8) tells us that Moshe Rabbeinu didn't want to accept these tools of the yetzer hara. But Hashem told him that these were the most beloved gifts to Him because they were used to help build klal yisrael. It's also therefore fitting that the kiyor (which is made from these mirrors) is used to bring peace between a man his wife (who is a Sotah).