The most significant value I get from Costco is that the quality is consistently high and the options are well curated. I like not having to go through 100s of brands of basic staples like yogurt, toilet paper, milk, etc...
Personally I've found that "well curated" sometimes becomes a double-edged sword in Costco's case. Mainstream brands can be expected to be available perennially; however, smaller/niche items and brands will often get cycled out or replaced by something else which may or may not be comparable.
Many times I've found products at Costco that I enjoy, only for them to suddenly stop carrying them a week or a month later.
A typical grocery store will carry on the order of 40,000 unique products. Costco is hard limited to a max of 3,700 unique products. If a store wants to bring a new product in, something must be rotated out. This also means that if a product wants to stay in Costco, it has to sell a certain minimum number of pallets, per month, to be worth it.
Many times I've found products at Costco that I enjoy, only for them to suddenly stop carrying them a week or a month later.