In games, this works in a purely self-interest model with players out to either improve their own position or hinder another's; that's because in games there are 'winners.'
this is a method of communicating with other players. Some games require very little diplomacy (The Double, for example, can be played without contacting any other managers in the league, though if you want to arrange private sales or such, you will need to contact them). Others are very heavy on diplomacy such as WWIV, with up to 40 players all contacting each other each turn.
Diplomacy is a board game, war game, and strategy game set in Europe in the era before the beginning of World War I. Diplomacy was the first commercially published game to be played by mail; only chess, which is in the public domain, saw significant postal play earlier. Diplomacy was also the first commercially published game to generate an active hobby with amateur fanzines; only science-fiction/fantasy and comics fandom saw fanzines earlier.