This is an exercise done before pranayama which helps to release the neck muscles and free the breathing passages for a more profound experience during pranayama. I also find it very useful for keeping neck pain at bay. Since I have fused cervical vertebrae and have had whiplash twice in my life, my neck can cause me a lot of grief...doing this simple exercise does help.
On the esoteric level, Brahma is one of the 3 gods of the Hindu trinity that is responsible for the creation, preservation and destruction of the world. Brahma has 4 heads and see everywhere at the same time!
Oh and here is a nice link about Brahma if you are interested in Indian mythology.