Some of the MRE's look pretty decent, but are they really any different / better tasting than your typical grocery store canned or frozen food?
Not better tasting, just more expensive.
It seems like they might be good for camping or hiking...
The chief advantages of the MRE are logistical. MREs do not require cooking, heating, rehydration, special handling, special storage, and are lighter than their canned equivalents.
Napoleon famously said, "an Army marches on its stomach," and in his time that meant either carrying a tremendous amount of provisions (including driving livestock behind the soldiers) or living off the land by appropriating food from locals - perhaps with a promise to pay them for it later, perhaps through outright theft. Napoleon also, somewhat less famously, was responsible for the invention of canned foods - he offered an award for the best method of preserving food so that it could be carried with his army without fear of spoilage. The winner of the award came up with a canning process which was less than optimal given the fragile nature of glass jars, but metal vessels were soon substituted for glass, and modern food was born.
Other methods of food preservation predate canning, of course, such as drying meats and vegetables and the use of salt to preserve meats. These methods were also used for combat rations, but since drying, salting, smoking, and canning are very time-consuming (and therefore expensive) processes, the preparation of fresh food on site long remained the chief way that soldiers in the field were fed; indeed remains the chief method for most of the armies in the world to this very day - US forces are a notable exception.
The US Army originally used the same methods as Napoleon did before canning: they carried fresh food (including livestock driven behind an advancing force) and appropriated food from local farms as necessary. By the Civil War there was widespread use of individual combat rations, mostly hardtack and bacon/saltpork. Although these could be carried and eaten on a march, they were not all that palatable, made for monotonous fare, and did not contain enough nutrients to keep an army healthy for long. Again, most of the nutrition that Civil War soldiers received came from food cooked in the field and carried with the army on wagons or mules. The most important improvement to military logistics by the time of the American Civil War was the presence of railroads which meant that most supplies need not be acquired locally. (There were important exceptions to this, to be sure.)
Combat rations further improved during World War I with the development of trench rations - meals scientifically designed to provide necessary nutrition and with packaging impervious to the environment - an environment that included poison gas weapons. But, once again, most of the meals consumed by combat troops were cooked in the field, mostly using ingredients transported long distances. The major difference was that trench battles in World War I lasted far, far longer than they had in the Civil War. This meant that even fresh cooked meals had to be transported right up to the front lines, and various methods were devised (or improvised) to do that.
The necessities of World War II and the experiences from the previous war had spurred the Army to develop new methods and equipment to deliver fresh-cooked meals to troops on the line. The insulated mermite container was one of these developments; one remembered (oftentimes with a mixture of fondness and disgust) by veterans from nearly every war since because it remained in use for the nest half-century.
The standard way of feeding soldiers on or near the front lines, or in bivouac away from permanent garrisons, was to cook food at a field kitchen, load the food into mermite containers, and transport the food to where it was needed. Food was served directly out of the mermite containers onto stainless steel mess kits (two plates plus spoon, fork, and knife) that each soldier carried. The containers were then transported immediately back to rear areas to be cleaned for the next meal. Mess kits were cleaned on site, after eating, at a line of five trash cans: The first can was for food waste, and the other four held water and gasoline-powered immersion heaters (pre wash with warm-soapy water, wash with warm-soapy water, boiling water to sterilize, and a warm water rinse). The wash water could often be discarded on site, but the trash would have to be hauled away for sanitation reasons.
This entire process, though efficient compared to what had been done historically, required a lot of time, effort and resources. There was not only the manpower, equipment, and fuel needed to transport food to the kitchens, but manpower, equipment, and fuel for cooking, transport to the front, setup of the mess kit cleaning stations, transport of the empty containers and trash back away from the front, and the cleaning of the containers, pots, and pans.
By the 1990s, however, all that was history. Gone are the kitchen tents and cooks, pots and pans, and the seemingly eternal mermite container. Gone also are the mess kits and the immersion heaters and trash cans needed to clean them. Instead, when troops in the field are not eating MREs, they are given food from tray packs, served on paper plates, and eaten with plastic utensils. Like MREs, tray packs are precooked and can be heated simply and quickly when needed. The tray packs are transported in insulated containers, but, unlike mermite containers, the reusable insulated tray pack transport containers do not come in contact with the food and therefore do not need to be cleaned. Since the paper plates and plastic utensils will be thrown away, no field sanitation is needed other than trash bags and a Humvee to take them away.
