Traditional inboxes are often used for personal messages, or for work purposes pertaining to a single person. They tend to have only one user account associated with them. They can usually only be accessed by one person at a time and, therefore, are not adequate for larger-scale professional situations.
Shared inbox tools, on the other hand, allow you to have multiple users using the same mailbox, each with their own credentials. They can all view (and reply to) messages received by the shared inbox’s address, as well as compose new messages on that address’s behalf. received by the shared inbox’s address, as well as compose new messages on that address’s behalf.