I dont even see how these numbers help a postman. Its great if you have a computer to pathfind you there but without it, utterly useless as far as I can see.

Nonsense. Normal addresses are just as useless unless you already know the area. If you are hell bent on using plus codes without technology you could create a high resolution plus code grid overlay on a paper map. Map grids are already an established concept in military maps. It's leagues ahead of "grey shed behind the old shed that used to stand on bob's property" kind of directions we're talking about here. Furthermore, there is a pattern to plus codes you could memorize and actually be able to orient yourself arbitrarily in relation to other plus codes, given sufficient IQ.