I'm not so sure it can be a letterbox. When registering companies with the Handelsregister in Germany, I think there is a legal requirement to have a physical address. At least from my experience.
In reality, just like in the Paradise Papers leaks etc, an individual accountant's office might have 4000 of these lined up next to their door.
Of course, for the vast majority of businesses, this is not much of a hurdle, and for most consumers it isn't all that helpful either, but end of the day, scammers are gonna scam, whatever the obstacles are. Even those that aren't outright scammers, will be paying accountants to have a plaque on their door to show that this is their "physical office" ...
Hopefully such efforts at least help courts assign some blame when the time comes.
Luckily for a few euros per month you can rent quite nice addresses and the relative mail forwarding services (e.g., once a day, once a week, etc.)
I should have looked up the definition, before using the word.