Microsoft Project - Tracking - Tracking Durations

Tracking Durations 

On the same dialog box, actual duration can be tracked.  If the only data entered is actual duration then the calculations being used will be.
Remaining Duration=Duration – Actual Duration

And

% complete = Actual Duration/Duration.

By only inputting the actual duration, problems in the schedule are not highlighted in advance.
Whereas if the actual duration and remaining duration are input then possible problems show up earlier

Duration = Actual Duration+ Remaining Duration.
As the duration is calculated, this will show variances against the baseline.