Okay, Mick. You asked, "If this is a conspiracy...why?"

In order to enable a hugely angry and fearful public reaction in the USA early in the Bush administration which would further enable the launching of one or more unprovoked foreign wars of aggression in the Middle Eastern area, which wars would advance the interests of a group of neo-conservative planners, most of whom are deeply entrenched in the oil industry and the Bush administration.

And secondarily to that, a host of other factors could come in, but the primary reason would have been to put the public in a mood for war. Not a legitimate defensive war, but unprovoked attacks on sovereign nations which have not attacked the USA.

911 was not an attack by a nation on a nation. It was a criminal act by a clandestine group of conspirators (either foreign or domestic or both). As such, attacking another nation over it was not a rational or appropriate response. Since the strategic realities in the world did not make it even remotely feasible to provoke Afghanistan or Iraq into attacking the USA first (Why would they be that suicidally stupid? How could they even do it?), it was necessary to arrange some other form of attack and then blame the Afghans and the Iraqis for it by association.

Crude but effective. It worked. A mob is easily led, particularly when they are very angry and fearful. Just tell them who the "enemy" is, and they will go and kill him for you...or they will cheer while you do it, supposedly on their behalf.

A major provocation was needed to get the public onside for a war. That's why.

(And that's simply my theory. I don't know if it is correct. But you did ask the question, and that's my answer.)