EDIT: addressed at BAS
It's you who appear to be a stranger to reason given the sheer volume of evidence available, even discounting circumstance and even eye-witness statements. I refer to you my previous answer - do you have anything based on actual evidence rather that your inability to believe otherwise?
When discussing this topic I prefer to stick to the science based stuff because it is the hardest to deny, but seeing as you're asking here's a few reasons why it might have been done:
Put simply - profit and power.
Since that event, countless thousands (probably 100's of thousands) of people have died in wars which were made publicly palatable after it happened. This has extended the American empire (hungry for oil and power) and provided unimaginably huge profits for many, largely American, companies.
'Anti-terror' legislation has rescinded many of the freedoms our ancestors fought so hard for, to the benefit of those in power (look up the patriot act for starters). To the benefit of the power elite (keeping control of us plebs).
In the case of building 7, it housed several shadowy agencies - FBI, CIA, Department of Defense, Various Banks, the IRS, the SEC (a massive haul of case files relating to the Enron scandal were lost), the Secret Service and the Federal Emergency Management Agency. It's destruction would have been very convenient for getting rid of evidence relating to the planning of the event or any other evidence for that matter that conspirators wanted rid of.
It was known that the demolition of the WTC, which had been applied for, would be MASSIVELY costly, as explosive demolition had been ruled out. -
http://www.asbestosattorneyguide.info/the-demolition-of-the-wtc-was-refused-did-you-know-that.html - then there's the coincidence that the new building owner had just reinsured the buildings specifically against 'acts of terror'. He got 4.55 BILLION (look it up . . .).
In the case of the Pentagon, just the previous day Donald Rumsfeld announced that 2.3 TRILLION(!) was missing
http://www.youtube.com/watch?v=xU4GdHLUHwU So a good time to bury that bad news, not to mention increase the defence budget at a time when military spending was already out of control.
I don't claim to know why, or who knew or whatever, but the actual physical evidence for demolition is out there if people would only look. The two-hour ae911truth.org video is a bit boring but quite a good place to start.
In addition there is a HUGE amount of circumstantial evidence, for example the BBC announced that building 7 had collapsed 20 minutes before it even happened, with the building still standing in the background!! Explanation: confusion on the day . . .
http://www.youtube.com/watch?v=PNK1V6S2cbo
Did you know that in a scary neo-con document titled 'Project For a New American Century - Rebuilding America's Defenses' (http://www.newamericancentury.org/RebuildingAmericasDefenses.pdf authors include Wolfowitz, Rumsfeld and John Bolton) there is a quote "the process of transformation, even if it brings revolutionary change, is likely to be a long one, absent some catastrophic and catalyzing event – like a new Pearl Harbor". The plan for invading Afghanistan was before the president days before the event
http://www.youtube.com/watch?v=19tnCIBJtJQ
Now maybe I should ask you: Why did Hitler burn down the Reichstag and blame it on 'terrorists'?