This one FAQ from Shopify’s announcement hits the nail on the head for why this new section is necessary in a social media world that is increasingly mobile (emphasis added):
How is this different than the Facebook Store app I already use?
Existing Facebook Store apps don’t work on mobile devices, which is how most people now use Facebook. As well, existing Facebook Store apps only add a tab to your page, while the new Facebook Shop section appears as a larger section on your main Facebook Page. This means more people will be able to easily discover your products. Finally, the new Shop section includes a subscribe button that lets people get notified when you add new products.
E-commerce conversion rates on mobile are generally lower than they are on desktops. Giving people an easy way to buy things on Facebook without having to bounce between a social network site and a merchant’s own website should help improve that.
Facebook (and, as a result, mobile) is a focus for Shopify because the social network dominates social media traffic to its stores. A March 2014 infographic from Shopify said that two-thirds of social media visits to Shopify stores came from Facebook sources.