You might have an LOB application at your company that uses IMAP to pull data from specific mailboxes. In order to access those mailboxes you don’t need necessarily to have a mail-enabled user or a mailbox assigned to the account. In general it was enough to grant FullAccess on the shared mailboxes to the account, which might be a service account.
Well, it was sufficient. This changed with Exchange 2013/2016.