you heat the element, it will open earlier because it's hotter, but how does that help you get a more accurate measurement of the air temperature?
The aim of the design isn't to get an accurate measurement of the air temperature *at the time of the measurement*, but to anticipate what the air temperature will reach *after the heating is turned off*. As the radiators and the water they contain as used in most domestic heating systems hold a fair amount of heat which will be released after the boiler is turned off, the heat source (boiler) needs to be turned off
*before* the desired air temperature is reached, otherwise the set temperature will be exceeded. This is why the thermostat has either software anticipation or a hardware heater built in. In an ideal world, the preheat on the thermostat will be calibrated for each installation, but most of the time, the normal value as installed is "close enough".Blown air heating systems need a different control regime, preferably a proportional control system, which takes account of the heat lost by the building, so that the heating runs at a reduced power level at all times, in a steady state.