Mixers of this type makes bathing and other water treatments uncompromising safety. So, eg, in case of a sudden shutdown of hot water, Thermostatic mixer overlaps cold water, thus protecting the user from cold shock.
In the case of, If you turned off the cold water, the thermostat momentarily stop supply of hot water, thus protecting you from burns. It should be noted, that the thermostat does not preclude the possibility to use ice water or boiling water - just set the temperature knob to the extreme cold or hot position. As a result, the fuser tune to a particular water supply, and its user will get exactly, What did you want.