How heat works
It’s not scientifically clear exactly why heat is such a treat. However, a good guess is that there are several minor positive effects that add up to … therapy. Most of these effects are also beneficial in other ways. Not incredibly beneficial, and not even notably different from icing: for instance, a 2010 study showed quite clearly that both ice packs and hot packs were beneficial for neck and back pain, and about equally so.2 But a small therapeutic effect is still valuable even if it’s small, and there are probably situations where it works even better — after all, these were people with acute pain bad enough that they went to the hospital. They may have been a bit beyond the help of a hot pack!