Quoted:
It's recommended on queens now in general, but it's actually kind of critical to have a center support that runs parallel to the outside rails if you're using split queen boxes.
Much the same way that a King Frame will have a support like that to catch all 4 sides of each box.
Boxes used to be actual spring units that were mounted onto flat, hardwood frames. Nowadays, most are just "foundations", a wooden frame, that are usually made of pine so the extra support helps.
Speed
yeah, but what exactly does the center support actually support
if the box sping is only "touching down" at the sides, what does the center do?
I could understand if there was some hardpoint in the center of the box sping that was resting on the center support, but it doesnt appear so