The head and the neck are closely related, even more than we might think. Look at the image below, describing the nerve networks of the face and neck. A nerve is like an electric wire. When they bundle together, they form roots (equivalent of a cable) and then they connect to nuclei (equivalent of an electric box) before finally transferring the message to your brain (main computer).
The sensations of the face, eyes, sinuses, teeth, gums are transferred through the trigeminal nerve. Trigeminal means «three twins» because this nerve has three branches for different parts of the face (forehead, cheeks, jaw) that we call V1, V2 and V3. V is roman for the number five, fifth cranial nerve.
The sensations of the back of the head and neck come from the occipital nerves that merge into the cervical roots (C2, C3).
Now, the interesting part. C2 and V1 end up in the same part of the electric box in the brainstem. Also, C2-C3 and V2-V3 overlap for the ear and jaw zone.
That means that if someone has a pain in the eye or the forehead, it can be felt in the neck. It also means that if there is a pain in the neck, it can trigger pain signals in the eye and forehead. In the clinic, many people describe this «C2-V1» path, from the neck to the forehead. And it’s not surprising, once we understand how our nerves are wired.
This is also the reason why neck problems can contribute to headaches. If there is inflammation  or strain in the roots, bones and muscles, all this can trigger or increase a headache. On the reverse, a migraine  coming from the brain can also cause neck pain, even if the neck is perfectly fine. This is a typical example of the ping pong theory (See this post ).
For this reason, people with migraines and other headaches (for example after a whiplash) often seek the help of chiropractors, physios and massage  therapists to target the neck issue and help the headaches. (See this post ).
Now, this anatomic highway means that if we do injections in the neck, it could in theory quiet down a pain in the forehead. (See this post )
Summary: the head, the neck, the face and the jaw share the same pain networks. Pain in one place can come from another place. Targeting the neck may improve headache if the nerves of the neck are playing a role in the problem.
Bogduk N, Govind J. Cervicogenic headache: an assessment of the evidence on clinical diagnosis, invasive tests, and treatment. Lancet Neurol. 2009;8(10):959-68.