Why Might Bombs Have Been On Board the Planes?

I just stumbled upon a good explanation at The Movement site.

The planes were being controlled by someone on the ground or in another plane, either using existing technology built into the plane, or having planted additional hardware in the plane for that purpose.

First, if you haven't looked at the section on weird coincidences you may wish to read how someone in the entertainment industry had already come up with the idea of planes being flown into the World Trade Centre using remote control technology. To recap:

X-files spin-off "The Lone Gunmen" aired in March 2001. (snip) My theory is that the planes had some sort of device installed (like in the x-files plot) and an explosive to destroy the plane in the event of a failure.

(emphasis added)

Makes sense to me. Bombs on the planes could explain a lot about the strange, incredibly explosive crashes of all four 9/11 planes.