Acupuncture needles without coating are the classic tool in traditional Chinese medicine (TCM). For TCM therapists an acupuncture needle must respond to 3 functions: stimulate the acupuncture point, conduct the heat well for moxibustion and form an energy bridge between the patient and the therapist. In China, therefore, almost exclusively needles with a copper handle are used. Copper is known to be an excellent conductive metal for heat or electricity or just about energy.
If you look at a perfectly sharpened acupuncture needle under a microscope, you can see that the metal surface is rough. This roughness requires extra force during the puncture and causes pain. A coating would smooth this roughness, thus allowing for a softer and less painful puncture of the skin. But if you want to do moxibustion, or apply an electrical stimulation vianeedles, or do trigger point acupuncture, needles without a coating layer are sometimes better.