To get the selling fees back. In this case, it is a little odd, since the transaction is not actually going to be cancelled.
You can agree, refuse or ignore a request for a Mutual Cancellation. Agreeing or ignoring gives him those fees back. Refusing will not.
Both of you can leave feedback and Detailed Seller Ratings as usual. His easy acceptance of the return, in my opinion, would encourage positive feedback and the normal 5 Star Ratings.
You should be returning the original purchase with "tracking" in case things do not go well.
Do not leave feedback or Ratings until you have received and inspected the replacement garment.