A DC to DC converter (not "inverter") converts DC power at one voltage to another voltage. In an EV, it is used to replace the 12 volt battery and alternator of a conventional car, getting power from the high-voltage battery instead of from an engine.
12 volt DC power is needed for lots of stuff not related to the powertrain, such as the lights and radio.
You need one unless you also have a 12 volt battery that stores enough energy to get you through all of your driving until you charge it again at the same time as you charge the main high-voltage battery.
