Why? to register your car and other things. Many people have parents and family they can use the address of, others don't.
So far, paying someone to use their address or convincing a friend is the only way I know if you're in that situation.
I found a campervan business in the Netherlands who would register, insure and pay road tax on my behalf and we have a contract between us to say that I am the legal owner (because when you register a car with the government it actually means that they are the owner, and you don't want to 'give' your vehicle away in a swindle) - so make sure you have a contract between you and the person who registers it, if they're not close friends or family.
For mail, you will need an address, anything you can negotiate that suits you. Note that PO boxes are no good for registering a car, other government stuff or receiving goods, so you still need to pretend to live at a physical address.
