Root cellars used to be the way that people preserved their food in the old days. They are useful because they are usually at a stable temperature during the winter and can keep food from freezing, and during the summer they are cooler than room above the surface in a house, thus keeping food from going bad due to the heat. The humidity in a root cellar is also usually pretty stable. This is all because they are below the surface and this acts as a kind of insulation.
For those who are interested in cutting their energy consumption this may be an easy option for food storage. The effectiveness of this type of storage is not in question as it was used for much of human history until the 20th century.
The early chapters of Build Your Own underground Root Cellar, covers choosing the appropriate location for a root cellar and has tips on choosing the right soil and other methods to avoid having a cave-in. There is also a nice, thorough list of everything that a homeowner needs to construct their root cellar.