Scaramucci was a hatchet man- a person brought in on a short term basis to perform an unpleasant or unpopular task. Firings, layoffs, restructuring, etc. The person takes all the blame for it and then leaves shortly thereafter. It leaves DJT in place without facing the direct criticism for carrying out the task.
Whitaker is the short term AAG likely for a similar reason. Come in, do something Dem's and/or normies don't like and then leave. Barr comes in as permanent AG without the controversy.