That is a heavy duty solution for a trailer that may have had sideways bends when cornering with concentrated loads. A company I pulled for in the 90s did something similar with 2X3/8 flatbar on all their 48ft flats after one trailer came into the yard with a 5 inch sway to the right after turning left hard with a 50k counterweight centered on the deck. It was the only one of 20 48s on that fleet that ever had it happen in 5 years that I know of.
Just extra support, helps againt what's called a ladder shift. As @beastr123 said. Pretty standard and can be from factory or added on.
