"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.
As of the latest update 1.15a the mailbox gives 1 free stand storage which is very useful.
(Note: It does not give the stand storage anymore)