The SIM cards available through Best Buy cover talk, text and mobile data. Data only cards that you might use with a tablet rather than smartphone are still only available directly from Google. Getting a SIM through Google is free, but you'll have to wait for shipping. By putting the cards in retail stores, Google is providing a viable option for folks who would rather go to the store and get the SIM card immediately rather than wait. It is worth noting that Best Buy won't be able to help set up the SIM, though Google provides some pretty easy to follow instructions on the process.
Google's decision to put SIM cards in stores marks the company's latest effort to expand the reach of Fi. Late last year, the company announced it was expanding the compatibility of its service to work with most Android phones and iPhones, altough the latter experience is still limited.