Excerpted from Bensky:  Chinese Herbal Medicine Materia Medica, 3rd ed.

• Releases the exterior and disperses cold:  for exterior cold patterns with such symptoms as chills, fever, headache, body aches and pains.  Most commonly used when accompanied by dampness with joint pain, a general feeling of heaviness, sleepiness. or when there is pain the the occipital regions as in Nine-Herb Decoction with Notopterygium (jiu wei qiang huo tang).*
• Guides qi the the greater yang channel and Governing vessel:  to direct other herbs in a prescription to the areas served by these two channels.*