Having seen these a many times on the road, I assumed it had to do with refrigeration. Jalopnik has a quick explainer as to why it is not that. They do tell us that the diamond doors are popular with refrigerated trucks tho.
If you want to be specific, these are "diamond pattern stainless steel doors" according to Utility Trailer, the manufacturer. The reason they look that way is simple yet pretty clever: The quilted steel stops light from reflecting directly backwards. It breaks bright reflections and angles them away from drivers.
That's right. Truck drivers and tractor trailers are actually more considerate than the cursed bunch of drivers who cruise around with HID lights that are not properly dialed in for road use, and inevitably end up damn-near blinding the rest of us.
I would think a high tech thing called paint could stop reflections.