Certain purchases still require human help, including financial services as well as anything that requires an age check (think booze, cigarettes and lottery tickets).
Mobile Checkout isn’t universally available in NYC, at least not yet. It’s available in “participating” stores that primarily revolve around Manhattan, although availability does extend as far north as Washington Heights. Don’t be shocked if it becomes virtually ubiquitous, at least. Although 7-Eleven isn’t typically known for long lineups, this could lighten the load for cashiers and improve you experience in those moments when you just want to be on your way.