Arcpy.mp allows you to open project and layer files. The GUI allow you to save maps and layouts and layer files. We need to be able to open map and layer files in arcpy.mp to modify the update connection properties and other properties before loading the in the project. Having all the possible maps and layouts a users would want to use in a project is not practical if it numbers in the dozens and only a few are used at the time. This way you can build a library of mapx and pagx files to use in your projects.