[Christ]

The thermostat is there to prevent coolant from leaving the engine/heater circuit.

The thermostat doesn't just open once and stay open, like some people seem to think. It opens when the fluid touching it exceeds it's rated temperature, and closes back up when the temperature falls. It takes several of these events over a period of time to "warm up" an engine package.

You can usually see the first opening event visually, though, if you dont have a dummy gauge. After the first one, they're much less pronounced.

The first time it opens, it lets ambient temp coolant into the engine until the fluid touching the tstat is cooler than it's set point, at which point it closes and begins circulating in just the engine and heater again. Lather, rinse, repeat, except the average temp of the coolant being brought in gradually increases until the tstat just stays open.