As teams progress through an iteration, it is important to understand the state of the iteration. As dozens or even hundreds of tasks are worked on within the iteration, measuring progress and remaining effort (or, "to do") become critical. This is especially true on larger projects.
Within an iteration, the remaining effort represents the true state of an iteration at any given time. As the iteration progresses, the remaining effort required to complete the iteration's tasks should obviously be decreasing. Not all tasks will necessarily be identified during iteration planning. Some may only be identified as current tasks are progressed and completed. By consistently managing the total remaining effort of all the tasks, a very clear picture of the overall iteration's status is understood. If at any given point during the iteration it appears that the work is not approaching zero by the end of the iteration, then the team and/or management may elect to take appropriate measures, such as reducing scope.