I have often wondered of the difference between having a duration and an estimated duration in MS Project.
Apart from the question mark indicator next to the duration (as an indicator), is there any other benefit or difference between the two?
I mean does project take this into account for calculating schedules?
It is simply that, estimated. When you don't know how long the task will take you "estimate" the duration. Otherwise, it is a known duration. Project does take it into consideration when calculating schedules. It can also be good tool to help manage unknown task timing.
Microsoft article about Estimated (task field):
Description The Estimated field indicates whether the task's duration is flagged as an estimate. The Estimated field contains Yes if the task duration is an estimate and No if it is a set duration. You can change whether a duration is estimated or set by choosing Yes or No in the Estimated field.
Best Uses Add the Estimated field to a task sheet when you want to view or change whether a task duration is estimated. Add this field when you want to sort, filter, or group for whether tasks have estimated durations. Sorting, filtering, or grouping based on the Estimated field can be helpful when you want all the estimated duration tasks to be displayed together so you can change the estimates to set durations.
