One way an independent publisher could use FLoC is for "lookalike" targeting. For example, if your best customers often have IDs D4W1 or JQ82, you could choose to target ads at those IDs.

Some entity might choose to run a "store" like you're describing, but there's no reason there would only need to be one.

It looks to me like Chrome is proposing a low-level feature that anyone can build more sophisticated products on top of.