How many times have you been driving home and realised you needed a loaf of bread, some milk or whatever other staples you frequently run out of? Wouldn't it be great to have a drive-through convenience store where you could get those things without having to get out of your car? I would definitely have used such a store at least once, if it existed.
Now the down sides. The selection would be a little limited by necessity. The prices would likely be higher even than ordinary convenience stores. And if you're ordering a lot, you might need to stop to arrange it properly in your car before heading back on the road anyway. Still, it would be interesting to see someone give it a go.
Mokalus of Borg
PS - A quick Google search confirms I'm not the first one with this idea.
PPS - Still, it's not so common that I'd call it "convenient".