It just means that there will be mailings of some sort. I believe you can see the addresses in the member’s list for those who subscribed to that tier that includes physical goods.
The option to add a physical address is always optional for the subscriber, but not the author. They can opt out or back in at any time.
We suggest that if you plan on sending something in future, to select that option just in case and then subscribers can decide if they would like to participate. Thanks!