Buzsaki argues, in this book, that brain waves are the method that the brain uses to coordinate activity in different regions of the brain. (High-frequency waves coordinate the activity of small areas, low-frequency waves coordinate the activity of large areas.)

http://www.amazon.com/Rhythms-Brain-Gyorgy-Buzsaki/dp/019982...

If you're curious about brain waves, I highly recommend this book. It is fascinating.