A lot of times the time you were away from your computer is just as important as the time you were active. Maybe you were at a meeting or maybe you were helping your coworker with some other project.
The point is that since you were not using your computer, ManicTime can't offer any help as to what you were doing while away.

This is why ManicTime will display Add tag dialog every time you will return to your computer so you can tag this time while things are still fresh in your mind.