A variac is an autotransformer that is used to adjust the AC line voltage above or below the standard voltage. Since the device is an auto transformer it does NOT provide isolation from the source (line voltage) as does a standard transformer. Therefore if using a variac in conjunction with test equipment, you should add an isolation transformer in your setup.
Variacs have many uses in industry including hot wire cutting of foam, coffee roasters, and compensating for AC line drops in equipment. Variacs have been used for hot tubs/spas that are located at a distance from the main house electrical supply system.
One of the main uses for a variac is in servicing of electronics equipment. Equipment that uses vacuum tubes is typically turned on at a reduced voltage especially if the equipment has not been used for a long period of time. This allows capacitors time to “reform” the electrolyte slowly. It can also allow the vacuum tube filaments time to stabilize. I often use a Variac when servicing electronic equipment that uses transistors and semiconductor devices to allow me to run the equipment at a reduced level while monitoring current draw while I look for excessive heat build up in the semiconductor devices. After replacing an expensive transistor or IC, it is wise to slowly raise the AC input voltage to prevent burning out the replaced device due to an unresolved problem somewhere in the unit.