From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156846-391407
Question: Can you tell me why raised beds are so popular, and also
give me a little information about how to make them?
Answer: Raised beds are simply garden beds where the soil surface is
built up so that it sits higher than the surrounding area. You can
make a raised be by raking garden soil into broad, flat-topped areas.
Or, you can create a low box from materials such as brick, fieldstone,
or wood -- or specially made raised beds -- and fill the box with
topsoil. Raised beds help soil drain -- and warm up -- in the spring.
They permanently define a garden area so that you can focus your soil
amendments, such as compost, on that area, and also refrain from
walking on it and compacting the soil. They aren't appropriate for
every garden situation, though. For example, in hot, dry cliimates a
raised bed may dry out too quickly, especially if the soil is sandy.