1. No it's not a shortcut. It works even if you don't have the Facebook app installed. Some people either don't want the official Facebook app (for various reasons), or their phones don't have enough space for it. The Messenger app is smaller, and still allows you push notifications of your Facebook messages despite not having the Facebook app. One bonus is you only get message notifications and not the other notifications.
2. You can use it as an app for SMS, like Handcent or Go SMS. Group SMS through that app will go through Facebook servers so it will be free (you just need data), and when sending an individual message, you can either send it to their phone number or their Facebook account. The chatheads feature make this convenient as you can SMS with someone without going out of the app you are currently in.