Imagine you're on a trip in another country that has different power grid voltage and probably also different plugs than your country. None of your devices will work.
But Hey! Gladly you have the brand-new Power Grid Voltage Converter with you! A small box where you can attach the right plug at the grid side and get maybe 110V and you can plug in your device on the other side and get 230V (or vice versa) out of the box, so you can recharge your phone, razor or whatever you need.