Image a scenario: a workflow is used for multiple teams - A and B. Team B start to work after team A finished thier job. It means, when a ticket is moved to team B, it is "Done" for team A. But because the workflow is used for both team, status of ticket is not a "Done" status. When we view Sprint Report for team A, all ticket are "Issues Not Completed".
It is very helpful if we can choose which statuses are "Done" statuses when we generate report, the same as how we choose Rapid View and Sprints