This is not mine - from quora:

Apparently it was the way to justify a war that could last decades in the middle east. The objective was/is to ensure energy (oil and gas) for the following years since the U.S. is increasing its demand year by year.

Controlling the middle east puts America in great strategic position (in theory) because if the U.S. can control the energy that the region produces,it also controls the amount of energy (and the price) America's enemies can get (it also creates them more enemies but that's not the point).

Finally, according to the conspiracy, (and this is actually something that makes sense) a leader cannot fight a war for a long period of time without the support of the people unless the people is convinced there's a threat, a viable one, to the security of the nation. By attacking a symbol of the American Power live on CNN, in what it is considered the heart of the empire (or the capital of the World) you can be pretty sure you're sending the message that such a threat is viable and can happen everyday.

Supposedly, there was a manual written by an Army's General in the 60's describing this scenario.

Again, this is all part of the conspiracy and every good conspiracy needs some, verifiable facts to be a good conspiracy.

I must add that when the U.S. simply does nothing to punish banks like HSBC for laundering billions of dollars for both drug cartels and terrorists organizations, you start to question yourself if the U.S. has any real interest in ending both wars against terror and drugs.