"I am the Mailman and my mail is superb!" —The Mailman
The Mailbox was added to refund the people who lost their data after some updates in the New Universe. The people who lost their data with proof recieved a free "Yo-Yo" which gave Killua to the people who lost their data for free.
On update 1.15a the mailbox gave 1 free stand storage which was very useful.
(Note: It does not give the stand storage anymore)