These rations are pre-cooked and are usually packed in a 'flexible' tin or pouch.  The longevity of each ration depends on the manufacturer and county of origin.  On average, in good storage conditions it can be expected that the a ration will last 2 - 3 years.

The advantages are that the ration pouches are ready to eat, no preparation is necessary, just open the top and eat.  They can be heated and its sometimes better to do so.

The US MRE's are made for military personnel but contain everyday items, like skittles and tootsie rolls.  Each pack is a meal ready to eat, and contains a large amount of calories.  A cooker is supplied with each pack (unless stated otherwise, Tuna being an example of a pack without a cooker), which makes eating this kind of ration even more convenient.  Just add the pouch and water to the cooker and it will heat itself. The latest MRE's come with hot beverage pouch where the coffee / tea is added and it is placed in the supplied cooker to be heated.

Wayfayrer rations are military supplied rations in a civilian wrapper.  They are identical to the ration pouches supplied to the UK military (and others around the world).