The thermostat REGULATES the water cooling..
It is not a matter of "Holding Back" the water circulation if you will, because it is a heat issue, not a timed issue..another words, 180 degrees is 180 degrees no matter how long it takes to achieve it..
You have 2 temps going for you, slightly hotter engine coolant, and the slightly cooler radiator water...the thermostat is what regulates the amount of cooling the coolant receives...Thermostat opens, the system stabilizes at a temp, then when below the cut off, closes and cools the water..
Without it, Cold Engines would run cold longer, (not good for it and parts that will wear, and unburned petroleum byproducts) and then Engine heat as it rises, would become additive...If you pull a long uphill grade, for instance, the heat would continue to build without regulation, until such time as pure airflow could bring the temp down..(which may be miles) which may be over 230 degrees by the time it starts back down..
Inversely proportional, If you run a long downhill grade the coolant may run as low as 125 degrees..not good for the engine at all...
The thermostat provides a short "Window" of cooling for the engine to keep it mostly constant..say 190 to 205 (ideal op temp) Which is why you don't see the gauge doing large excursions under different driving conditions...Also without this "regulation" a host of different things wouldn't work properly, like the radiator cap for instance, (constantly blowing off pressure/water)
The thermostat holds that "Window" of temp to keep the engine at optimum operating temps..The net results are better Fuel economy, lower emissions, better internal (oil) operating temps..
If the whole radiator runs hot during the high cycle, than that heat is transfered to the cooling side of the Transmission cooler..for automatics..meaning that fluid will also run hotter, cutting the useful life of the transmission..somewhere I read (may be true, may not..) For every hour the fluid runs hot, it cuts 10 hours life of the gearbox..
So yes a Thermostat is needed for a Better running engine..