So speaking from India, while I haven’t mapped any developer tags myself yet, IMO they would actually be very useful and it was actually something I was looking for in the recent past. Frankly I would argue that developer would be even more important than architect, builder, etc in India.
India is going through a major development phase and in every major city nearly every new building (residential or commercial) has a big fat developer prominently attached to it – also in very literal ways. The physical signs are everywhere on their projects, and if they’re one of the bigger names they will have advertisements across the city as well as promote it on their websites.
Example of a couple big names advertising on their websites:
This is one picture I took on ground recently where the name is prominently advertised during development
This is one picture I took on ground recently where the name is prominently advertised after development -- notice the 'Parishram By Rustomjee', so Rustomjee are the developers
One use case for me is effectively using developer as an additional filter – in India we extensively use names for buildings; however this leads to a case where multiple buildings, sometimes even from the same city have the same name and using developer could be an additional identifying filter.
Old buildings might not have this information and it would simply be blank, same as quite a few other OSM tags.
Or maybe you want to look at all projects by a certain developer and want to view their amenities/features as mapped on ground, and don’t want to rely just on official websites filled with corpo-speak?