From cooling the engine. When you are standing still and the engine is running it consumes about 1l/h. I just looked up some numbers for EVs: 100kWh battery, heating takes 1kW for every 10K temperature difference, so 3kWh in -10°C. Its higher if you use additional stuff like the heating for the seats. With 150kWh/100km consumption you lose 20km every hour you are in the heated car. I would say that’s a noticeable difference compared to no heating. I also checked how much an AC takes in summer and its about 1 to 2kW for 30°C.