The Google shopping that exists today is not the same service that was originally launched. It used to function more like a standard search, but limited to all online stores. So long as there wasn't a robots.txt, Google was index the site and include the product in their shopping results. So basically like standard search, but with logic to determine if a site represented a store.
Then Google modified shopping so that it was opt-in for the retailers, meaning a large number of stores were no longer included. To be included in Google shopping now a retailer must have a Google merchant/adwords account. It's more of a curated search now.
Nothing wrong with it now, but it used to be great for finding hard-to-find items or items that were mispriced by less sophisticated retailers.