Boiler Plate has a higher tensile strength than mild or regular steel and so it is less prone to rupturing. That is needed in a pressurized unit, like actual boilers and old steam engines.
The industry has thrown the name around (boiler plate) as if it was needed when it is absolutely not needed. It is of no use in a non-pressurized unit. Our 1/4″ high quality mild steel fireboxes probably have more tensile strength than the others with their “boiler plate” fireboxes that are the same thickness and thinner!