Once a parcel is on its way, an address change does more harm than good. This setting draws exactly that line.
With Only validate unfulfilled orders active:
We recommend it for practically every store with fast fulfillment. It prevents the frustrating case where a customer edits their address hours after the label was printed, and the change never reaches the parcel.
If you fulfill slowly (pre-orders, made-to-order), the companion setting Set time limit to edit shipping address (default 30 minutes) gives you finer control over how long edits stay possible.