Why are the heating pipes of the old Jetta all connected by four-way?

Water flows through the heater tank through the engine. The heater pipes of the old Jetta are all connected by four-way, and the water flows through the heater water tank through the engine. There are two valves on the engine block, which are respectively connected with the intake pipe and the exhaust pipe on the heater. Just open the valve after connection. Jetta is the automobile brand of FAW-Volkswagen Co., Ltd., a joint venture of German Audi AG in China. Its first product, Jetta, was launched in Europe on 1979.