Healing skill affects the amount of healing you are capable of. You may not notice the effects as much at lower levels, but KT uses something I call evil aura. It's a measure of how evil an area is, and it gets worse as you go deeper. It affects the amount you can heal as well as increases the chance of failure for similar life-giving spells like raise dead or resurrection. Your healing skill, class, alignment and guild all increase your resistance to the evil aura and improve the odds of successfully healing, raising people from the dead, etc. Without it, you will have a very difficult time in D16, which is an extremely evil place.