Linko64: User acquisition
It's been their main goal since day 1, the problem they face is turning gree users to paying users. Every giveaway is purely down to user acquisition and flipping to those obtained users to paying users. Epic has a tough job doing that. If i recall their buying user conversion rate was less then 10%, maybe even less than 6%. But in the eyes of certain corner, it's at least a obtained user who may go on to purchase things at some point
This, user base and brand recognition is probably more important in modern market than profit itself.
Also, every game they give away is one less game sold through Vapour (and GOG). Not exactly 1 to 1 ratio of course, but plenty of people do actually browse the Epic library before buying games.
Having and running the launcher instead of "I don't like FortCraft anymore so will uninstall the launcher forever" is a excelent way to make people play their games. And lastly, the less said people will spend on single player games the more micro$ will be bought.
It seems to be working very well, since they went from 6 months -> 1 year -> forever weekly giveaway.
Edit: Seems to be working pretty well the 2nd, since Vapour is giving away hardware to try lock people there. GOG will suffer the most :(