I wonder why this order to destroy the visual evidence.

Could it be because the visual evidence shows the attack to be a false flag, a staged attack, or a faked attack orchestrated with crisis actors ?

As I previously pointed out, the numerous photos available in the British media show no blood in the street, no blood on the truck, and no damage to the truck that allegedly has impacted 186 people. The police are shown standing exposed to gun fire a few feet from the truck cab firing into the cab that allegedly contained an armed person. Police do not take this kind of needless risk when confronting an armed suspect.
http://www.paulcraigroberts.org/2016/07/18/nice-brings-to-mind-operation-gladio/

The demand for destruction of the visual evidence is suspicious, to say the least. The only obvious reason for the demand is suppression of evidence.

Remember all of the surveillance cameras with recordings of whatever occurred at the Pentagon on 9/11? All of the recordings were confiscated. The official story is the recordings show the airliner hitting the Pentagon, but no recording has been released which shows the evidence that the government claims to have.