To keep them out of the way during welding. Ironworkers work around dangerous equipment that is heated to very high temperatures--hot enough to liquefy iron. They also have to use their hands constantly, so it's crucial for them not only to avoid burns, but also callouses and blisters on their palms. When metal gets too hot, they can easily grab it with their bare hands or tools--such as long-handled pincers or tongs--that are insulated by leather or thick rubber grips. The tucking of one leg into the boot helps prevent burns on the shin by keeping another layer of cloth between the skin and riveted seams on jeans legs. It's also more comfortable when kneeling, because there's more material between the knee and riveted seams.