World Trade Center 7 - September 11, 2001

I'm more open to conspiracies than I used to be. There is a large list of things which were considered outrageous conspiracy theories at one point which turned out to be true. That said, I don't understand what the motivation would be for our government to do this. What's to be gained from knocking down a third building that couldn't have been accomplished from just knocking down the twin towers? It seems like a huge risk, and kind of short-sighted, to run two planes into the towers and then knock down a third, relatively insignificant one with a controlled demolition. Why?