I think in this case the reason for the password was to avoid accidentally messing up the calibration. On the FX-888D it is WAY too easy to mess up the calibration—press and hold the up arrow and you think you're adjusting the temperature, but you're actually changing the calibration. There are two levels of password protection, IIRC: one just protects the cal, the other completely locks the iron to a fixed temperature.
In theory, the password feature is a good idea if you're a supervisor who doesn't trust your technicians to use the correct temperature, but if the part you're soldering is that temperature-sensitive, reflow is probably the only correct way to do it anyway.