Sometimes it’s a club which formed as a merger of others (Newcastle United, for instance), but often it really doesn’t mean anything in particular. Late 19th and early 20th century habits were to give teams names, be it ‘City’, ‘Town’, ‘United’, ‘Albion’, or whatever. Man U, Leeds and West Ham were all just relaunches/rebrandings under new names of previous teams.
And, to take that further, D.C. United is just an attempt by an American team to sound European, just as they now have Real Salt Lake, despite the fact that America has no king to grant the team a royal charter, and we have F.C. Dallas, for no apparent reason. :rolleyes:
Maybe I’m not understanding the OP, but I take such names simply to mean something like “Manchester is united in supporting this team” (not actually true, of course), or Manchester [comma] united.