A member of your team was provided with a dedicated user mailbox and therefore an Office 365 License. Now the team member has separated from your organization or is on an extended leave, the License can be released.
In order to keep the email for future reference and/ or for the same staff member to return, converting the user email to a shared mailbox is the best option. It would then be possible to unsubscribe from the Office 365 license.
Microsoft has a nice Article describing the benefits and limitations of shared mailboxes.