What is the purpose of 'Due Date' under an action added to a Help Desk request?

Summary: What is the purpose of 'Due Date' under an action added to a Help Desk request?

Content (please ensure you mask any confidential information): I created an action plan and added the action plan to a help desk request. Then, I clicked on view details of an action (Actions and Plans --> Task --> View Details). There is a due date under the action which I changed, but it is not overriding the due date calculated on the basis of duration added while creating the action through 'Manage Action Plan Actions'. What is the purpose of this due date