Cheap smart trainers do not have accurate power meters built in - for example the famous Tacx Vortex Smart.
Many riders have accurate powermeters fit on their bikes.
Today, Zwift cannot use the powermeter readings to control resistance of the trainers during power targeted workouts.
The idea is to use the power readings from bike power meter to verify that the smart trainer power readings are OK.
The smart trainer resistance should be corrected based on the bike power meter readings.