If there were actually bombs setup in the WTC to facilitate its collapse...

Why weren't they detonated at the time of the aircraft impacts?

This would make the most sense. If the aircraft were guided into the WTC by radio beacons, as many bomb theorists contend, then why not collapse the tower as soon as the plane struck?

Doing so would pretty much erase the bomb theories altogether.

The fireball would cover up any of the debris from explosives.

The only possible reason to wait to collapse them would be to mitigate the loss of life. If the buidlings were rigged with bombs, the conspirators of such a plot might look at the loss of life in casualty numbers as a liability to the maintenance of their story that planes and terrorists alone collapsed the buildings. If the casualties were too high, there would be too many concerned parties involved, and their questions would expand exponentially throughout the population of the country. Less casualties, less interested parties (and less of a payoff to keep them from enacting lawsuits).

But besides that, why wait to detonate the buildings and bring up the question of detonations vs. a natural collapse ot the buildings due to fire? If the buildings were rigged, they could be theoretically detonated at the very moment the planes impacted.

What possible gains could be yielded by waiting to detonate?

They needed the Fires to explain the collapse.