If the illness you are treating has a significant psychosomatic component, and the patient "believes" in the treatment being offered, then there is a likelihood that the treatment will be beneficial. Placebos have been shown to not only help in psychosomatic symptom control but in "real" situations. Pain is certainly real, although there is a large component of how much the patient focuses on the pain which is reflected in the rating of its severity.
A Double-Blind, Randomized Multicenter Trial Comparing Duloxetine with Placebo in the Management of Diabetic Peripheral Neuropathic Pain - Raskin - 2005 - Pain Medicine - Wiley Online Library
Here is a study showing benefit in hip and knee pain in patients wearing magnetic bracelets. The improvement was 26% and there was a placebo group wearing a non-magnetic bracelet which improved less but also improved. The benefit of magnets was equal to that which patients obtained with exercise or Celebrex.
Randomised controlled trial of magnetic bracelets for relieving pain in osteoarthritis of the hip and knee
Now there is no explanation of why this might work. And there are many studies showing absolutely NO benefit so this one might be a fluke, like flipping a penny 5 times and getting heads each time. It happens but is meaningless.
Therapeutic effects of magnetic ... [Complement Ther Med. 2009 Oct-Dec] - PubMed - NCBI
I like the conclusion of the abstract for this study:
"CONCLUSIONS:
"Our results indicate that magnetic and copper bracelets are generally ineffective for managing pain, stiffness and physical function in osteoarthritis. Reported therapeutic benefits are most likely attributable to non-specific placebo effects. However such devices have no major adverse effects and may provide hope."