Battle.net has had some tumultuous times over its existence. This November 30 Battle.net will turn 21, and like any service that old it's evolved and grown with the times. It started as basically a chat bot and matchmaking system and has changed over time as the industry changed. To be fair, some of those changes were in response to it, or could even be said to be in mimicry of it -- lots of companies over the years have taken the basic idea of Battle.net and run with it.
Right now, it's fair to say that there isn't a single Blizzard game that doesn't at the least contain a strong multiplayer component.
This one's a bit to unpack.