If you had an opportunity to peer into any well-equipped mid-20th century audiophile's equipment locker, you would most likely find a variac. And what is a variac? It is a variable output transformer that permits adjusting AC voltage from 0 to more than 130 volts.
My own variac is a General Radio Corporation type V10. It's so old that it was made before the days of three-prong AC, but still works perfectly. Actually I have two variacs. The other is built into a Monster Power AVS 2000. In the AVS 2000 the variac's output is controlled by a microprocessor designed to keep it's output at a constant 120 Volts. I use it to control the ac to my projector. Besides keeping the voltage constant it also shields the projector from spikes and brown-outs.
But who, in the 21st century, really needs a variac? If you use any tube electronics, older solid-state gear, or have equipment that goes for more than six months at a time without being turned on, YOU need one.
I use my variac for several essential tasks. Its primary job is to save me money. It accomplishes this by preventing me from blowing up gear. I use my variac to power up anything that I haven't used in a while. Instead of flipping the on switch, I attach the component to the variac. I set the Variac to 0 volts output, turn on the component attached to it, and then I gradually turn the variac up to 120 volts. By doing this I spare any older or less than fully-charged capacitors in a component from getting a nasty voltage surge, which could cause them to fail. By gradually re-forming the capacitor, it can be restored to a complete level without stress.
I also use my variac whenever I first power up a piece of tube gear that hasn't been used for a while. Again, a gradual voltage increase saves the internal parts from the shock of going from going from off to on. Also when I re-bias a tube component I usually put it on the variac to insure a steady, repeatable, AC voltage during re-biasing.
Another use for a Variac is with an audio component made for the Japanese marketplace that has been configured for 100 volts rather than 115 or 120 AC. With a Variac you can simply dial back the voltage to 100 volts and your Japanese-rated component will work just as it did back in Japan. Problem solved, without going into wire-switching or transformer-swapping.
So, if you don't already own a variac, you might want to keep an eye out for one in your EBAY, flea market, and garage sale, travels. They usually go for short money (because most people don't know what they are) and are one of those tools that every serious audiophile needs in their equipment closet.