This damper was added to the outside vent duct line as an additional block against outside air intrusion. Under spousal orders I had just changed the microwave vent to the outside (the proper way to go, but much more difficult to accomplish) but then there was some "flowback" when the wind was pressing against the back of the house (westward facing), and I was ordered to reduce this flow before the winter arrived in full force. There is this same damper on the microwave unit itself, but I hoped an additional block near the outside exit would significantly reduce that reverse flow. We haven't had much wind yet to see if it was worth the effort.
What I must state clearly is that the order/delivery process from Appliance Parts Pro was astounding to say the least. Finding and ordering the parts on-line was cake, and I chose the el-cheapo ground shipping. WOW, the parts arrived the NEXT DAY. I was in no particular hurry for this project, BUT surely appreciate and applaud the super quick turnaround. If a "critical" kitchen appliance were broken and unusable, fast parts delivery would mean everything!!