It is a principle in Anglo-Saxon law that destruction of evidence is prima facie indication of guilt.
Informed persons know that destroying evidence is a crime.
Therefore the US needed to find a mechanism whereby it could destroy evidence in the Mt. Carmel Center and plausibly deny it had done so. The US also had to fabricate a story to cover the deaths of the Davidians whom it had already killed, or intended to kill. It provided such plausible denial. Alternately, the Branch Davidians could be blamed for the deaths and the fire. But even after the event the perpetrators needed a cover for further alteration of the crime scene.