1. Appeal to developers by making exclusivity deals with them (e.g: we give you a bag of money upfront if you only sell your game via our store for a year) which is really beneficial for developers as the game business is quite risky. The developer gets guaranteed cash and Epic gets everyone who would buy that game on their platform (minus the people who are outraged by the exclusivity deal)
2. Appeal to the customers by giving away free games every week. Epic pays developers for you, so you get a free game while the developer also gets paid. Epic wants you to have a library on their platform so you keep coming back, and because you need to visit the store to claim the free games they literally train their customers to use the store, from there it's a small step to convert them to paying customers with good deals.
It's absolutely baffling to me Google didn't do anything like this, they have literally more money then they could spend, yet they didn't do anything to get people to actually use their service. Compare it to the Gmail or Chrome launch suddenly there was this 'free' product you could jump onto, it was technologically superior and exciting and it came out of nowhere. Imagine if they did that with Stadia, just suddenly release it without fanfare, all your favorite games are on there and you can play X hours a week for free or something like that.