Think of them like rigid zip-locs that you can also use for mise en place, to store spices and grains in your cabinet, individual portions of leftovers for eating at the office, vinegar and vegetables for pickling, and perfect portions of stock for freezing. Along with anything else you'd use a zip-loc in the fridge for.

The big thing about delis is that you'll simply save a lot more food, because they're so extremely convenient.

What size of deli cups do I need mass quantities of?

Do you use the tiny ones that take-out dipping sauce might come in? Or the ones that an order of hot-and-sour soup might come in? Or the ones that an entire Chinese entree might come in? Or the ones that a half-gallon of soup might come in?

I'm guessing your answer is "all of the above" but I'm curious.

I know where you're coming from, though, because in the last couple of years, without even consciously trying, I've noticed myself getting more and more use out of our used take-out containers. Either we were clued out before, or there's been an evolutionary leap in the quality of deli cups, such that they're now better than disposable Gladlock-type plastic containers, which in turn are better than my aging collection of Rubbermaid and Tupperware containers. They're even arguably better than my snap-closed glass storage containers, which I've liked for years because they can be reliably cleaned.

The cups I use are about the size rice comes in from a Chinese or Mexican restaurant, or half the size an order of hot & sour soup comes in. Think large coffee mug.