A great use for Office 365 shared mailboxes is to convert a staff leavers account to be able to refer back to it easily. The account will stay in place for as long as you like and can be visible to 1 staff member or a team of people.
Email can still be delivered to this mailbox for checking if emails are still being sent there. All passwords are removed from the account so there is no worry of your previous staff member still having access to the account. There is no ongoing cost to an Office 365 shared mailbox however there may be an admin cost to set this up.