2000 Ford Taurus Poor heat!

Hi all. 2000 taurus 3.0L OHV, 172000km. Heat has been hit and miss, often blows only lukewarm air. Lots online about this. The thermostat is fine, and the engine reaches operating temperature in reasonable time. The blend door seems OK too. I flushed the heater core while pinching the bypass hose, and this SEEMS to have fixed it... So what is the purpose of the HEATER CORE BYPASS HOSE? I see no water control valve to cut off the coolant flow to the core when the heat is off, so why is it there? Wouldn't we all get better heat if we eliminated the bypass hose all together???

Thanks all.