The point for having an upstream GNOME installation built from sources is if you are going to build an extension, a theme or a GTK App and take advantage of all the new features of next GNOME. However it is also useful if you want to help GNOME to get better by submitting bugs ..or it could be useful if you are just curious to see what’s coming next 🙂
The tool for building GNOME is nothing else than JHBuild.