I had never seen this aircraft before, and was really interested to see it when xoxoxoBruce sent the images of it along. This is a Super Guppy, and its job is to move aircraft sections from point A to point B.
Because once you've started to build an aircraft, what do you do when key parts are to be added at another plant? You need an even bigger aircraft...
One that can actually open its jaw like a snake...
Big enough to swallow and cough up most of a V-22 helicopter fuselage, and to fly it halfway across the country.
I did not know this but the first "guppies" were built for the moon program. Now there are several, apparently; googling for a while showed that Airbus has a few of them too.
It would be frightening to see this in the skies above you.