This is used to open an existing Ez3d project using a file selector. The project file contains all the information about the current state of an Ez3d project, including objects, lights, cameras, and so on.
You can also open an existing project by dragging it from the desktop file manager into the Ez3d viewer. If the file is not an Ez3d project, but an ordinary 3D file, Ez3d will try to load it into the current project.