Many people buy cheap cars at auctions, pay cash, and give a false name and address.
They never fill in the form to say they own it so the car is driving around with no owner.
So they need no tax, no insurance, no MOT, and if they get caught speeding there is nowhere to send the ticket to.
You can only get done if the DVLA know you are the owner, if you never declare you are the owner you can drive for free.
This is why it important to report ANY car you see with no road tax as they probably have not got insurance either. Also important to give the address of the person who drives it.