Why the Diffuser at the Neck?

Mindbodygreen.com said so well:

Topical application of essential oils offers numerous therapeutic benefits that complement healing achieved via inhalation. Essential oils contain compounds from diverse chemical families, which give them unique analgesic, anti-inflammatory, anti-spasmodic, and mucolytic properties.

And because of this, applying to the back of the neck, where your cervical spine is (home base of the nervous system) is one of the most effective. Because this is just  inches away from your nose, you are provided all sorts of aromatherapy benefits. As an added benefit, because the central nervous system is where you receive information from your senses, create and store thoughts/emotions, the oils work on a molecular level to "speak" to the nervous system. The nervous system, then sends out the information to the peripheral nervous system, sending out the messages to your brain and spinal cord. It is a kind of information highway.