Bulk fermenting a dough for eighteen hours at a cool room temperature lets wild yeast and lactic bacteria work through the starches slowly, which does two things a quick two-hour proof cannot. It breaks down more of the gluten structure into simpler compounds the body finds easier to digest, and it builds acidity that acts as a natural preservative, which is the actual reason our loaves keep for four days on a counter rather than going stale by the second.
A lot of bread sold as sourdough in supermarkets is a standard yeasted dough with some sourdough starter added for flavour, fermented for a couple of hours like any other loaf. That's not dishonest exactly, but it isn't the same product, and it won't behave the same way in a sandwich or freeze the same way either.