All of the Quick Connect attachments create pressure on the water. Unless an o-ring is added to the spout, the water will leak around the connection of the spout and attachment. The Quick Connect O-Ring slips over the end of the spout and seats in the o-ring groove just above the Quick Connection. With this addition you are assured of a solid seal between your Quick Connect attachments and the spout. You may need a Seal Pick Tool to help properly seat the o-ring.