Having said all that, I suppose it might be used one final time at the passport office after the passport has been packaged up, just before it is collected by the courier company. That's why it's left there, but why that specific position in the passport?

No, it's just a guess. It doesn't seem logical that the passport office would leave it in place if it wasn't required to be there once it has left the passport office. And if it's not required once it's been delivered then it seems it is required to be there during the delivery process. I wouldn't imagine the delivery company needs it for tracking, they have their own lables and barcodes on the parcel, The position of the yellow sticker in relation to the passport RFID suggests it might be working in conjunction with the RFID, that was my reasoning.