This one puzzled me for a while, because most Japanese didn’t strike me as the kinds of people who’d want every stranger passing in the street to know their names and where they live (especially if there name is Gomi like someone in my street!) It turns out the reason is because several houses in the same street/ block have the exact same postal address and so the only way post and guests can reach the right one is by reading your name outside. This is also why post that is addressed to you sometimes doesn’t get there if you don’t stick your name outside (and JPLT refuse to send you anything if it isn’t) or c/o someone whose name is there, although all these things are less true for flats (apaato, mansion, coopo etc) as it has a distinctive building name and then an individual room number.
Getting back to my original query on why the Japanese would be happy to have that system, despite seeming to be very private at times, privacy is not a Japanese system (hence the use of the borrowed English word in Japanese), and certainly not privacy from your neighbours!
The Japanese street and home (non) numbering system is explained someone else on this blog somewhere- one of the pages perhaps??