based on their webstore and their butting with Apple/Google, they want to be the one stop shop for desktop and mobile. desktop was the obvious first step, but the aspect of a premium mobile game store is an interesting premise. And probably overly optimistic. I'm sure they will simply bend to the f2p GAAS model as much as Apple/Google did and simply want that platform cut.
There's also the matter of its own Unreal ecosystem it's building. It's letting kids use real (but watered down) UE tools to mod fortnite, which leads to future devs that make games in UE, which they can then publish to the EGS. Short of actual hardware, they seem to want full vertical integration of the game development process. And if Fornite money keeps flowing, I can see a 5+ year plan where they cover the hardware aspect as well.