If you want to average multiple sensors, including the internal thermostat sensor, then yes, you need to select your thermostat again. If you only want to use the thermostat and no other sensors, then you don’t need to enable the remote sensors.
when the mode changes because the temperature reading is above a certain temperature, then the heating set point is also adjusted (cooling is optional). When the temperature goes below the threshold, a cooling set point is changed (heating optional).
I found that changing the mode didn’t help when seasonsl change, so I added the temperature adjustments. If your cooling is set to 74 during the day, and the temperature at night drops, when you change to heat you want to bump up/down the heating temperature. Likewise for cooling…