Variac Is Not An Isolation Transformer
Variac or variable transformer (you can adjust the AC voltage from 0 to 240VAC) is use as a troubleshooting tool to slowly bring up the AC voltage so that the power supply components would not blow just in case if there are components shorted in the circuit. Applying the full AC voltage to the power circuit will surely blow the shorted components. Variac is different from isolation transformer because it is not isolated where else isolation transformer output is fully isolated from the Main AC.
Thus, it is important to have both transformers for troubleshooting purposes. One is for controlling the AC voltage while the other one act as a protection to you and to the equipment under test. If you need to get a set that have both the transformer in it then you should click on the photo above. It is an isolated variable transformer specifically designed for electronics troubleshooting