[roel_v]

Yes: efficiency and keepimg temperature stable at a certain point (ie, with a small hysteresis).

Ad 1: the gist is that it takes less fuel to heat a liter of water from 30 to 31 degrees, than it does to heat it from 60 to 61 degrees.

Ad 2: with a lower temperature, it becomes easier to control your overshoot. Actually this is related to 1, but what you want in the end is to have a system that runs at as low a temperature as possible, all of the time.