Where I grew up in the Pacific Northwest you would see these ALL the time on trucks, mostly due to the bad weather with a lot of rain, snow, etc. I mean, I used to go camping and would sleep in the back of my truck under the canopy.
In the south you never used to see them as much, but they're growing in popularity.
I had an Aluminium one on my 94 Toyota pickup, at the time I was on a tactical response team and I needed a dry place to store gear in bolted down lock boxes, so the shell over top of the bed with dark tinted windows kept the boxes out of site of curious eyes.