If you have a job that requires you to use power tools when out and about you may need a way of accessing power to run or charge your equipment. Sometimes electricians struggle on site where they may not have power or may have to turn it off to carry out certain jobs. If they have plug in drills they may not be able to carry out the work unless they have an onsite generator which they can plug in to.
Some people fit an inverter in to their car or van which converts the 12 volt power that comes out of the car socket in to 750 or 1000 watt for example. This will then allow normal house hold plugs to be plugged in to the invertor allowing them power and to charge batteries off the car battery. If you are using an inverter then try to do so when the car is running, but if you do use it without the engine running be sure that you do not run flat your car battery.