I just stumbled across wiki pages encouraging the use of an “object:" namespace in parallel to "addr:”, apparently created to assign addresses to things that don’t have an address but are near one: Key:object - OpenStreetMap Wiki
Trawling through the Wiki history it looks a bit like this was one person’s hobby horse rather than anything with any sort of community consensus behind it; is anyone aware of any wider discussion having happened to establish object:*?
According to the wiki history it is quite clear that these keys have been introduced by a user whose hobby was to tag the historic gas lamps in Düsseldorf with some kind of address without using the address: tags.
Meanwhile the object: tags have been used some 100k times spread all over Germany (~ 99%) with some uses in the neighbouring countries as well.
There have been severeal discussions dealing with these tags (additionally to those mentioned by @hfs ):