In the initial stages, the patient must be able to focus on both the pain and the EMDR stimulation simultaneously. It must be stressed that once the patient notices their pain, and attends to the bilateral stimulation, that's all they really have to do. Not everybody can do this easily. A full assessment is necessary prior to attempting to use this method with chronic pain. Finally, sometimes, pain persists, the therapist can do their best to remove blockages and stimulate processing, but EMDR cannot remove what is ecological.
Pain can be 'necessary' for many reasons.
It needs to be remembered chronic pain often comes with peripheral nociception, as well as neural phenomenon. The longer pain has continued, the greater the changes to the nervous system.