Apart from building various electronic gadgets, my main hobby is repairing broken things. These "things" at some point will include mains-powered devices. Take for example the old amplifier in your attic, your broken power tools or your neighbor's broken TV.
While repairing these devices you need:
-isolation (and one hand in your pocket :P)
-varying the AC voltage in a clean way (no dimmers please)
This calls for an isolation transformer: a simple 1:1 ratio for the windings but galvanically isolated, and a variable autotransformer (a variac).
An old UPS was repurposed for the casing.
The problem gets bigger (and heavier) with increasing power demands. The end result has already put a big dent on the shelf it's sitting on!