Why you use saturated steam in steam tracing?

If you use saturated steam, the main source of the heat is provided by the condensation of the steam at a constant temperature. The energy provided by the condensing steam is significantly more than what you can get from just changing the temperature of superheated steam. When the temperature remains constant, design calculations are also a lot easier; the temperatures stay the same (or almost the same) for the steam along the entire run and the pressure stays the same (or almost the same).