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.