While reading my copy of “Household Discoveries and Mrs. Curtis Cook Book” published in 1908, I found an interesting section on thermal retention cooking. They called it fireless cooking and even list recipes and instructions for their use. After reading this, I have been doing research on the history of their use in the U.S and have found them to have been quite common. They were even sold in the Sears catalogs. If you want to research addition information on old ones look under the names hay box or fireless cooking.
The book offered some interesting reasons for using them beyond the obvious one of saving fuel. One main reason was that you could cook foods that normally gave off a strong odor like cabbage, onion and sauerkraut without the smell. The odors are sealed in the box. This could be very useful if you were trying to be secretive.
On hot days, it limits your stove use and keeps the kitchen cooler. With ours, we have also found that you can use them for improvised coolers.
The book also made a suggestion that seems so obvious now that I have heard it. They suggested that if you have extra room in the box after putting in your food that you stick a bottle or pan containing boiling water alongside for extra heat.
My wife has been cooking for years with a wonder box/oven and now has a Saratoga Jacks thermal cooker, which she loves. Here are some links to blogs we have written in the past on this subject. http://Saratoga Jacks Thermal cooker, http://the wonderful wonder ovenbox, Wonder Box Oven recipe, Wonder Box Oven Cooker, Wonder Box Cooker, a Must Have
Personally, I think they are a must have if you are serious about preparedness. One you understand the principal you can improvise one almost anywhere.