Actually, it can be beneficial for both comfort AND healing. Moist heat is commonly used as a healing aid in various parts of the body.
Heat increases blood flow, which speeds up healing. Moist heat penetrates deeper than dry heat, even if the water does not reach the area of infection/inflammation.
However, what is unique about a fistula is the type of problem it is. Since a fistula is lined with epithelial tissue, the body does not necessarily view it as something that needs healing, but more like a new organ. So even with antibiotics, sitz baths, surgery, etc., your brain's and body's idea of healing may not be the same thing. While sitz baths may speed up fistula healing, they probably do nothing to close it.
Hope that doesn't make it more confusing.