Well to me the motive becomes secondary after it becomes obvious that the attack was not done by Al Qeada (at least it's obvious to me). But as to the possible motives, I will throw out a few.
The main one to me is to make the region unstable enough so that China does not use it as an energy corridor. China has been trying to use that region to transport energy to itself. As any hegemonous state that would like to maintain its hegemony, US will try to restrict other nations from become as powerful or close to as powerful as itself. I believe that might be one motive.
Another reason I saw on loose change was that there were several billion dollars worth of gold - some 160 Billion - in WTC underground, that were snapped up around 9/11 (I can't remember if it was before or after 9/11).
Another reason would be to turn the public against a particular group. A publicly known example of such is CIA's attempts in the 60s to launch false flag terror attacks on US soil as a reason to invade Cuba. Do attacks on local population, blame it on Cuba and the public will believe you, and use that as pre-text to attack Cuba for whatever motive you have. This is publicly known, unclassified by US in the 90s, and not something I am making up. And here again, the motive becomes secondary because it's obvious they wanted to do it, so using the motive as disproving it becomes futile (of course you can't disprove this one, but the same logic can be applied to 9/11). The motive is not too clear here either, but we know they wanted to do it.
And this reason above - to turn the population against a certain group - to me makes most sense, because there has been consistent rise in anti-muslim sentiment throughout western world since 9/11. This, I believe is engineered and not natural.
http://www.mlive.com/news/detroit/index.ssf/2009/12/flight_253_passenger_says_at_l.html
^^ These types of false-flag attacks are just meant to compound the sentiment.
Why they would want to turn their people against muslims? I believe there might be several reasons. Right now the sentiment is not strong enough, however its growing. They might be preparing for something in the future, and for that they need people on their side. It might be to show the people that they and Israel are the good guys, and those muslim countries are the bad guys.
So there's plenty of possible reasons. However, the problem remains that the real motive will only be known by those who planned it. All I am doing is speculating and throwing out some possible motives. It could be that they are using 9/11 for some long-term interests of theirs.
But I will go back to my main point. As with the Cuba example where the motive is not clear, it becomes secondary as to what the motive was. All we need to know is that they did it, and once you know they did it, why they did it becomes a secondary question. Of course you want to know why they did it, but you do not use that for disproving their guilt, because it's clear that they did it. Their guilt has already been proven, so whatever motive might be, you accept that as the reason they did it. Here the motive will not be known until the US government itself says that the attacks were false flag, and until it does that what I say will remain a conspiracy theory as well. Although I don't call it a conspiracy theory because there's evidence to support the theory, and when that happens its no longer a conspiracy theory.