The Advanced temperature control algorithm of BrewPi uses both environmental and fermentation vessel temperature (multiple sensors). The problem this solves is the air surrounding your fermenting wort will always change quicker than the wort.
Depending on where you put the sensor from a STC-1000 the liquid will either warm/cool more than expected because the controller waits for the correct temperature. The BrewPi will learn and predict how the liquid vs environmental temperature affects each other and shut the cooling/warming off early in order to hit the target temperature closer.
From the landing page: http://www.brewpi.com/
BrewPi has an advanced temperature control algorithm that can keep your beer temperature stable within 0.1 °C.
Instead of just heating when the beer is too cold and cooling when the beer is too warm, BrewPi calculates what the fridge temperature should be to keep your beer temperature in check. It then controls the fridge temperature. This prevents fluctuations in the beer temperature.