One thing it does well is providing only as much of said ingredient as you need. I recently tried to get into Chinese cooking, and before I made my first two or three dishes I had to run out and buy Hoisin sauce, chili garlic sauce, oyster sauce, Chinese rice wine, and minced ginger. That's quite a bit of an initial investment before I even decide whether I like the food or not. So if I can pay a little extra upfront just to get whatever amount of those sauces I need while I'm still in the trial phase, that would be nice.
As someone else said, local grocery stores could probably do very well just by providing their own meal kits. Wegmans does this on occasion, although their stuff is usually assembled to the point where you simply bake it.