You've had the option of chatting with stores in Facebook Messenger for a while. Wouldn't it be nice if you could complete the entire sale without leaving chat? Shopify thinks it can help on that front: it's trotting out a feature that lets you browse and buy from stores using its platform while remaining strictly in Messenger. You don't have to visit a website just to pick an item or check out. You'll likely have to start the conversation from a company's Facebook page, but it's entirely possible that you could stick to chat from then onward.
The move helps stores, of course, since it may simply be more convenient to shop from an outlet where you already have a chat thread going. However, it could be particularly convenient to you if you regularly purchase through your phone. You don't have to navigate a cut-down website or download a store-specific app -- you're using the same interface you use to talk to your friends. While there are certainly things a website or app can do that Messenger can't (you won't get elaborate product tours), it's good to have options.