So, its a little weird, but...
I used it once a number of years ago using Prime31's Game Center plugin. What it does is facilitate two (or more) copies of the game communicating with each other in a serverless multiplayer context.
What that means is: one of the clients is also the server (they are the "host" and act like a server for all of the connected clients).
You can do multiplayer this way and for some games it works really well (things like Space Team, Spyfall, or Codenames, where 'cheating' isn't important because the game is short and fast paced and switching to a different person as host if someone's cracked their copy is easy.
What you can't do is any thing that requires:
- A lot of server computation (e.g. lag compensation for a first person shooter: your host won't have the system resources)
- Any persistent data storage (e.g. any sort of RPG with stats and items, as changing hosts means restarting)
- Lots of players (you're going to be harshly limited by the network capacity of the host)
- Out-of-line-of-sight multiplayer (Game Center is set up to work on single-wifi-network or bluetooth connections)
In my project I was making a Tower Defense game and used Game Center to let each copy of the game that was running know about where different towers had been placed by the other people playing and give a damage, rate of fire, special effect, or range boost to towers placed in the same spot. So a poison tower placed by Player 1 would mean that Player 2's bullet tower in the same position got a small DOT effect. If Player 3 also placed a poison tower, then their tower and P1's tower would do slightly better poison, and P2's bullet tower's small poison effect would increase (the bullet tower also applied a small buff to the poison towers, but I forget what it was).