What is the need of the Facetime app in the MAS?
As you said, FaceTime for Mac only started shipping built-in with macOS Lion 10.7, whereas the FaceTime app on the Mac App Store runs on macOS Snow Leopard 10.6.8 (which does not have it built-in).

The Mac App Store download allows customers unable/unwilling to upgrade past Snow Leopard to FaceTime with Mac users on 10.7 or newer, or those on iOS.

As to why Apple does not tick it off for you, it’s simply that this isn’t exactly the same app that ships with 10.7 and above. Case in point, you can’t easily uninstall the FaceTime app that ships built-in, but you could uninstall the one from the App Store, just as easily as you’d uninstall any other app. The version from the App Store also hasn’t been updated in years, as evidenced in your screenshot.