So a cash-strapped game developer can go into the final stages of their game launch pre-paid and knowing they're already fiscally on track before it's even out. For indies that's a no-brainer.
That's assuming of course that Epic chooses to accept your game in the first place. My major issue with discussions regarding the EGS is that everyone discussing it talks as if every developer has access to what it's offering, which is not the case even in the slightest.
My understanding is that right now Epic's submission pipeline is very manual, there's not a lot of self-servicing. So they are focused on submissions which move the needle the most for Epic: Exclusives, free titles (also available to indies), and then AAA games. I believe they've suggested the wider release of EGS for more developers should be sometime this year.
The EGS is much more akin to a console ecosystem. You don't have to use it, except perhaps for the occasional exclusive. But lots of people feel that there is value there.