Point of League: The Battle

Okay, so I made a thread a few days back asking about why do some champs even exist in the League of Legends, considering they had no point in joining. I learned about the retcon, where it's League without League; where the Institute of War, and a lot of the basic foundation that established a purpose of the game, some of the champions, and even the players themselves in a sort of meta way.

Now, considering that the retcon will drastically change all this (But why even keep the map's title of Summoner's Rift? I digress), my question is, what is bringing all these champs together into one fight? There's no League (lore-wise) that brings them together, so with that lore gone, there's no reason for rivals (say and ) to be in the same team, or certain champs in general (like ) to exist.