And you know those really stupid looking cars where the driver clearly believes they are capable of going 300mph, and as such, has carefully installed a spoiler on the rear?
No, wait. Don't laugh yet.
You know those *really* stupid ones, where the driver has decided "hey, if it works well on the back, it should work even better on the roof!"?
Now you can laugh.
Or, I don't know, maybe it was a solar collector. The owner of the truck was clearly a believer in "bigger = better" - it was large enough to power, say, the running lights.