Hi Robert,
The driver for the feature was choke input supplies fed by vacuum tubes - these will warm up over a few seconds rather than "spike" straight away and it's a good way to soften the start.
There are other soft start schemes, i.e. old schematics for linear amplifiers using a resistor in the line feed which is shorted by a relay after a couple of seconds, or the modern way of doing this with NTC thermistors. However, PSUD doesn't do an accurate model of these.
Regards,
Duncan
