WatfordFC:Well, some games, such as battle boat variations, Crazy screen chess and Screen chess which need the players to set the starting position of the board are mostly played as private games so that a player doesn't have to change the start position so often.  But why games such as dark chess or Atomic chess should be private, I don't know!

I consider most of them to be games played by cheaters and with their own duplicate accounts.  Of course they want the games to be private so that others cannot see their games and the way they have cheated.