You either follow Abel Danger or the 911 truthers or others which I haven't bothered to look into. Each of these "theorists" have very different beliefs as to the motive. Let us assume the worst case scenario, that there was a "cover up" of some sort. There is nothing we can do now to change things. Gotta move on.
My belief is that there isn't really any mystery, theorist create their own mystery out of coincidences....it is that simple.
Both of these groups are filled with diagnosable paranoid people with nothing better to do with their time.