The average conspiracy theorist starts from what they want to believe (generally that the US Government/The Bavarian Illuminati/the Jeeeeewwwws/Citibank/Oprah Winfrey is responsible for all the evil in the world) and work backwards from that.
What they never do is stop to consider "If I were so and so, and I wanted to achieve aim such and such", how would I go about it?
For instance, suppose I'm George W Bush, it's early 2001, and for some reason I've decided that I want to invade Afghanistan. (Possibly so we can build that oil pipeline which last I heard still hasn't been built.) What do I do?
Well, the most obvious thing to do is to invade Afghanistan. Circa 2001 there were already some pretty decent reasons for wanting to go to war in Afghanistan in order to take out those objectively-dickish Al Qaeda (who had already attacked the US on a number of occasions) and the Taliban (who were just plain assholes). The US public could probably have been convinced to go along with this, even in the absence of another terrorist attack. We went along with Obama to Libya, and with Clinton to Haiti and Iraq (Desert Fox, remember) and the Balkans.
But let's suppose they decided that this wasn't enough, and that it was totally worth killing thousands of US civilians for no particularly good reason, in a conspiracy involving hundreds or thousands of other ostensibly-patriotic US government employees, despite the fact that if anyone ever found out about this then they'd all be executed for treason and the fact that everyone involved in this conspiracy has every incentive, before the attack happens, to blow the conspiracy open, save thousands of lives, and become a hero.
At this point I'm gonna skip several paragraphs from my originally planned response, because I'm getting tired thinking this hard about this much stupid. I'll ignore the "wouldn't it make more sense to pick another target" bit and the "surely a smaller scale attack would have worked just as well" bit... and even the "if you're already crashing an airliner into a building WHY THE FUCK would you also need to go to the ridiculous expense and difficulty of doing a controlled demolition AS WELL? Like the war couldn't have happened if we'd ONLY seen the crashing planes and the buildings HADN'T collapsed?
Instead I'll skip straight to my conclusion. The minimum bar which a conspiracy theory has to pass, in order for me to take it seriously, is that it has to make some fucking sense from the point of view of the people who are supposedly the conspirators.