Years ago, I had an aux water pump start leaking on a 190. Since I was in the middle of a ski trip and it was Christmas eve (no dealer within miles), I bypassed the water around it and drove without it. After I returned home a few days later, the pump was replaced under warranty. I could never tell any difference in the heating system, before or after. I guess it must do something but in the normal heater operation, it must not do much.