Because messing with mains-connected circuitry has too high chance of getting kicked and/or frying expensive parts. In the standard setting, the unit is a weak isolation transformer. Also, sometimes a higher-than-mains or lower-than-mains voltage is needed, e.g. for calibrating voltmeters (with or without additional resistive divider or regulated dummy load) or for playing with triacs and thyristors at lower voltage safer and more comfortable to mess with.
The unit consists of three transformers, with dual secondary windings. The windings can be connected in series or in parallel with attached switches. The combination of the windings selects the output voltage.
The third transformer was added as an afterthought, for boosting the output to about 500 volts. Its secondary is then connected to the series with the second transformer. As it was a circuitboard-mountable transformer, mounting assembly was made ad-hoc from nuts molded into epoxy putty.
The output voltage is VERY load-dependent ("soft"). For the purpose of this device, which is design and testing of low-power circuits and assistance in repairing of low-power power supplies, this is considered an advantage. The short-circuit current is less than 200 milliamps, and the output voltage drops to negligible.
For higher currents, the Lightbulb Limiter can be used instead (will not increase operator safety shock-wise but will protect the device under test from overcurrent), or a commercial 1:1 isolation transformer can be employed.
Inside view, towards front side
Inside view, towards the front side, detail
Inside view, towards back side
Inside view, towards back side, with front panel
Voltmeter - internal system
Voltmeter internal system, from behind
Voltmeter scale circuit
Voltmeter scale circuit, detail