A Dual Mode Thermometer consists of two temperature sensors in one: A Bimetal Dial Thermometer (for local indication) and a Thermocouple or RTD sensor (to send a signal). This redundant reading of a Dual Mode Thermometer increases safety and reliability.
Having two independent sensors checking the same point in the process allows you to do an informal accuracy check. If one fails, the other is usually unaffected and if you lose power, the bimetal will still function. And if the steam is bent, the sensor will still function.
In this video, REOTEMP Instrument explains all of the benefits to their Instrument Dual Mode Thermometers.
