Freeze-drying is the most successful method of food preservation.
Microrganisms like bacteria feed on matter leading to decomposition, and can also release chemicals that can cause food to taste bad.
Like all life, bacteria need water to survive. Freeze-drying removes the most water of any preservation method. Up to 98% of water is removed during the process, which is called lyophilization. To find out more about this process, read this post.
The other factor in the decomposition of food is the reaction of oxygen with enzymes in it. Lyophilization utilizes a vacuum and the finished product is also vacuum packed, so the risk of spoilage through exposure to oxygen is removed.
Canning, normal freezing and dehydrating do not remove anywhere near the amount of water or oxygen as freeze drying, and therefore the period of preservation is much shorter.
Freeze-drying significantly reduces the weight of food.
Water accounts for a large percentage of any food's weight. Fruit, for example can be over 90% water.
As lyophilization removes about 98% of a food's water content, you can reduce the weight of that food sometimes by as much as 90%.
For people having to carry their own food for days or months at a time during expeditions or endurance races, this is a huge benefit.
Freeze-drying retains the nutritional value of the fresh product, as well as its fresh flavour and colour.
Dehydrating and canning involves the heating of food to temperatures which can impare its nutritional value. The taste can also be affected and heat can denature and break food fibres changing the texture.
Freeze-drying is akin to keeping the food in a state of suspended animation, and so once rehydrated the food is as fresh and nutritious as it was the moment it was frozen.
Freeze-dried foods can be stored for the longest time.
Freeze-dried food can be stored at a wide range of temperatures without the food or packaging being affected, and the food remains fresh for many years.
So, in summary, freeze drying combines the convenience of canned and dehyrdated food with the freshness of frozen food. But there are also the additional benefits of having much greater longevity and being a fraction of the weight.