As soon as you want the external world to do something with your app (run workflows or read data), the Bubble API is the right tool. On the other hand, If you want your app to read external data or trigger some action in another system (like fetching a Facebook profile picture or sending a SMS with Twilio), the Bubble API is not the right tool. Instead, you should use a plugin or the API connector, or build a new plugin to add this new service.