filling in space between frame and bed

I just returned from a trip to Southern Mexico (where my retired mother lives). As you might expect there are lots of Toyota trucks on the road there (and many other makes). In many of them I noticed that the space between the frame and the bed of the truck has been filled in with solid pieces of wood. I wish I had taken a picture as its difficult to explain, but imagine the curved part of the frame from the cab to the front of the axle extended up to a the bed all along the frame. The fame appeared to be a foot or more tall at the front of the bed due to this section being filled in. Even behind the wheel well, where the gap is only a couple inches, there was a piece of wood filling in the gap. These spacers were held in place with long U bolts. While I am sure this adds support to the bed, it sees like a lot of work for questionable benefit. I wasn't looking all the time, but when i noticed I'd say about half of the small trucks on the road that weren't new(ish) had these.

Is anyone familiar with the benefits of this? I am in the Northeast and adding wood to anything on my truck would be foolish as it would quickly rot. I could see this perhaps being worth while in the Southwest or other arid locations. (My mother's city is high altitude, near desert, but still has a 3 month rainy season). Do you think it would strengthen the frame or just the bed? The roads there and only half paved, and there are topes (speed bumps) everywhere, so any car or truck takes a beating. I am curious about this mod.