The small is great for snacks, for freezing smoothies mixes in, jewellery & whatever else you would need a smaller bag for
The medium and large are great for freezing soups, portions of other meals for one or two people. Wet swimmers at the beach, toiletries. I use them all of the time for freezing broth ice cubes, frozen fruit that I buy from the bulk store, taking salad with dressing. Picnics, BBQ's, school lunches.
The XL 4 litre pouch is amazing for preparing marinated meats, cut up fruit, salads in a large quantity. Being totally leakproof they can be easily transported without worrying about spillage.