As it has been mentioned before I'm sure, this is the current plan:
Main focus is Epic, with its world and player factions competing, creating their purpose.
But ALSO, to have an option to "jump in" to game modes similar to Battle (and other game modes), when a player needs that kick of some quality fighting and insta gratification.
IMO it fulfills whatever mood or type the player is having. If you like to climb to the top of, or develop/organize a faction, or merely support it in various ways. If you like to craft and develop your unique character, all the tools are there. If you just enjoy a random good battle or siege the game should give that too.
As to how these game modes interact with each other, I think (personally), that battles that are inconsequential and for fun, should have a minor to no influence on your Epic character, but that there should be some connection between them. We would want players to use both game modes, but not strictly require it.