@aravindhrs extensions are camunda wide on all elements. So would say there is no specific use case for extensions.
Remember that extensions are at the Task Definition level, and are not dynamic per instance of task. So the values you place as configuration metadata would have to be retrieved through a script during the creation (or a Task Event) of the User Task, you retrieve the BPMN XML of the User Task, and then get the extension element, and get the Camunda Extension Properties array. See Get Extension Properties - Helper Script for a example