You may have a situation where a room is a long distance from the source of heat and is always cold. To mediate the problem you would consider installing an electric duct heater with its own thermostat.
It depends on where the thermostat is for the main air handler. The space could have higher heat loss/sq.ft. than other spaces (corner room, lots of glass, poor insulation, etc.) If the other spaces controlled by the main thermostat are satisfied and the problem space is still cold ==> Duct Heater required.