Epic announced in 2018 that it would make the multiplayer systems it developed for Fortnite available free for every developer to use, and today it's completed that task. The full software development kit is now available, providing the "essential framework" for friends lists, matchmaking, lobbies, and other cross-platform multiplayer services for games made in any engine and distributed on any store.
What Epic is offering is divided into two parts: Game Services and Epic Account Services. The Game Services are things like matchmaking, lobby, leaderboard, and stat tracking systems. The Epic Account Services are for developers who want to let players sign into their game and find friends using their existing Epic account.
Why is Epic giving its multiplayer technology away? For one thing, it can. "We built these services for Fortnite and are now operating at enormous economies of scale," Epic wrote in an FAQ, which is another way of saying that you can give stuff away when you've made as much money as it has.
Of course there's a long term strategy, though. Aside from "encouraging wider adoption of all of Epic's offerings," if other developers accept Epic account logins in their games, the company's launcher and store gain another potential user, or someone who already has an account gets more use out of it. To be clear, though, there is no requirement that says developers must integrate with Epic Account Services to use any of the other multiplayer functionality.