I know I know, #firstworldproblems ... but some markets like where I live in Orlando, FL, driving is a pain in the arse (not a walkable city), so I just pony up the $5 for the digital rental to save me the hassle.
Maybe you don't buy groceries, but I do. I need to eat, so I buy food from a grocery store. Conveniently enough, my local RedBox is located at my local grocery store. There's always something I can buy there to restock my refrigerator or panty. And if I wanted to watch RedBox movies, it'd be very convenient because I could pick it up there, along with some food.
If driving to your local grocery store is such a pain, you might want to re-evaluate your choice of city to live in. I've lived in a bunch of different places around the US and never seen a place where just getting to the local grocery store was that much time or trouble. (Work, sure, because the commute can be long and rush-hour traffic congested, but not the grocery store.)
I feel that queuing up a guaranteed watching experience from Amazon is worth the hassle of an extra few bucks compared to getting in my car and driving somewhere. (Yes, I assume I could walk or ride my bike. I've never actually been to a nearby RedBox so I don't know how far it is.)
Still possible for it to not have a movie you want, but at least you'd know that before leaving the house.