This isn't the first time SRAM has challenged our drivetrain perceptions - most of their component groups these days seem to be centered on it, in fact. Doubletap makes absolutely no sense when you first try to explain it to someone for the first time, but somehow it works. In the same way, eTap functions with a right shift operating the rear derailleur to a smaller cog, a left shift moving the rear derailleur to a larger cog, and both at the same time moving the front derailleur to wherever it's not.
I'll let SRAM explain what they've done from their perspective:
Okay, so yes, Shimano did offer a similar idea with the user customization of Di2, but SRAM has taken it a bit further in their complete ditchery of all that is holy about the traditional antithetical shift levers. Think about how often you used your respective shifters on your last ride. What eTap has done is bring a kind of balance to the road bike cockpit. For the first time, your shifters can finally be seen as equals. Left, right; both shifters work in harmony to get you where you need to go.
It's an interesting move from SRAM seeing as their 1x revolution seems to be accomplishing the opposite, putting all the emphasis on a single shift pod, but for road bikes it just make sense to keep that extraneous lever on the left.