Why do you always write the opening time on the mailbox? Who is it for? What's the use?

Hello, it says that the unpacking time is to remind the sender when to unpack the letter and show it to the sender. Unpacking personnel shall collect the mail at the designated mailbox (box) according to the specified frequency and time, and shall not advance or lag behind; Each mailbox (box) should be opened in the order of unpacking route to ensure that it is not left behind.