Flight time limitations vs Duty time limitations

[Lemper]

Some national authorities impose a flight time limitation, i.e. 8,9 or 10 hours max flight time in any 24 hours period, and this limitation dictates the use of a third crew member on long haul flights, notwithstanding the duty time limitations, while many other countries stick to the duty time limitations regardless of the flight time. Some airlines in countries where there are no legal limitations on flight time still use those limitations as a guideline for the crewing of their long haul fleets. As an expected consequence, there are airlines flying a 12 hours sector with only two pilots, still respecting their 14 hours max duty limit.
My question is: What is the rationale behind the imposing (by authorities) or using (by ailrines) a flight time limitation on long haul flights instead of only the duty time limit.
The sticking to the duty time limitation is the naturally preferred choice of airline managements for obvious and understandable reasons, so no point arguing this one; however I would appreciate authorized (and others too) opinions and arguments on the other.
Thanks.

[737incognito]

The Duty starts 1 hour before scheduled departure, so if there is delay before first sector it's included in duty time.
Flight period starts with first off-block so if there was same kind of delay it was not included.

I worked with both systems. In the second one management didn't care to delay my pick-up if delay was before first departure. They only cared about disruptions enroute.

It was not long range ops, but 3rd world country with an old (1969) regulation.

[The Sea]

What is the "Augment Crew" means ?

The flight depart 23:00 with 5 hours flight time.
The flight will back to origin at 10:00 with 11 block hour (12:30 hours FDP)

The allowable FDP is
11 hours for 1 sector
10:15 hours for 2 sectors

Is it possible to add 1 augment crew for FDP extension to 14 hours to perform this flight turnaround ?