Destiny 2 for PC isn't going to be released through Steam, PC's largest game store. Instead, it's exclusive to Blizzard's Battle.net platform. Destiny 2 is using Battle.net for purchases as well as for online gameplay.
It'll be the first non-Blizzard game to rely on the platform in that way. Battle.net is known for being home to Blizzard titles like Starcraft 2, Overwatch, and Hearthstone.
Battle.net is a reliable platform for online functionality, though some people will surely dislike the game's absence from Steam.
Bungie is still going to be in charge of maintaining the game's servers.