What happened on 9/11 was the culmination of many years of desire by Al Qaeda to inflict massive damage on America, using the most iconic places they could find.
America is a financial giant = WTC
America is a military giant = Pentagon
America is a beacon of democracy = US Capital Building (a safe assumption for the 4th target)
Having already tried to knock one of the twin towers into the other and failing on February 26, 1993, UBL and his merry band of morons came up with another plan, using knowledge they've gained through experience.
Experience says that a hijacked aircraft will land, refuel, and make demands. Passengers and crew on the aircraft have also seen this. This is what happens when an aircraft gets hijacked - period. (Pre-9/11)
Two of the planes hit the WTC towers. Both collapsed due to the impacts and subsequent fires. If one were to use common sense, one would imagine in such a scenario that the building with the most weight above the weakened portion would collapse first. Well, that's exactly what happened. One would also think "holy crap I didn't think THAT would happen" until about a half hour later when the other one did the same thing. At that point it became obvious (to the sane) that those buildings weren't designed to survive that amount of stress.
AA 11 was hijacked, and flown into the Pentagon. By this time, word has started spreading about what was clearly an attack, clearly not what we're used to when it comes to hijacking. This word made its way to the passengers and crew of Flight 93, who heroically decided that their aircraft would not find its target.
Terrorists are more than capable of finding weakness in their enemy, just like everyone else. These are not cave dwellers. They've hijacked enough planes to know what the procedure is. That was the information they used against us. 9/11 was mind-numbingly simple. Fly planes into the biggest buildings you can find, whatever happens next is just icing on the cake.