It's just about saving money (quantity). It's about QUALITY of the goods.
For example, some Costco stores will sell high quality New York Steak Shabu Shabu precut/pre-sliced, which I love for hot-pots and tabletop grilling.
The fresh meats (beef, pork, chicken, lamb) and fish at Costco and Sam's Club are a lot better quality than at Crests, Walmart, Target, and all other No-Fee grocery stores.
Another thing is the shopping experience/environment. A lot of people just feel a lot better/happier going to Costco and Sam's Club compared to all the no-fee grocery stores.
So both quality and quantity (savings).
In a way, it's kind of like owning audio/video/home theater equipments.
For some of us, it's not just about buying the cheapest things or the best measurement numbers. Performance is salient. But we care about aesthetics and pride of ownership very much. We want to FEEL GOOD and proud of what we own every day.
Oh, BTW, I have membership to BOTH Sam's Club and Costco.