Also known as camber. In steel construction, cambered beams are used a lot because with a camber, the beam used can be smaller and lighter than if a non cambered beam was used. I suppose this theory could apply to the strength of the beams of a trailer as well, a non cambered beam would have to be a heavier stronger material than a cambered beam.
Once bought an aluminum step deck with 17.5” wheels and returned it because with the dang arch it had a higher deck height under light loads than my combo step with 22.5” rims. There’s No point at all in owning a low deck aluminum step IMO unless you’re doing heavy loads all the Time
Dunnage in the middle is a requirement at some pipe yards. I just sold my 48' aluminium flatbed. I pulled a lot of pipe and casing loads to the oil patch. If you don't have dunnage in the center to begin the first layer you can be screwed as you get the fourth and final layer on the top. You WANT the trailer to flex under that load and spread the weight evenly.
The problem with putting dunnage in the middle of an arched trailer is that the load sits on the 2 most forward dunnage and the back bounces off. Or if the weight is on the front and back dunnage the middle will be loose. Make sure the dunnage cant move and remember the trailer flexes up and down going down the road. Not to mention it can screw up the weight distribution.
I found almost the opposite in my experiments with dunnage placement. By the time 48000 lbs of drill pipe are on the deck the camber of the trailer straightens out with centered dunnage. There's plenty of downforce on either end (I liked to leave about 6-8 feet of space from the front of the pipe to the front dunnage and the same for the back). I rarely had walking dunnage, but I put two straps on either side of the dunnage and added the penalty strap ahead of the front duo.