Do to a lot of things (citizenship, licenses, taxes, etc) you aren't legally allowed to be employed without a address on a form somewhere
Furthermore, most companies still exercise some control over remote work whether formally or otherwise. That someone can work remotely mostly from a fixed address in the same country, doesn't mean they automatically have the right to work from a beach in Thailand or as a digital nomad.
You still need a fixed mailing address for many things, so I assume they'd use that