Installing the Stand-Alone Addon Manager is totally redundant, since there's nothing there which is not already included in any of the full installers, and the Live Update is able to update everything.
The only single case when you might need to install the Stand-Alone Addon Manager, is if you want to fix problems caused by using OLD installers, those that didn't use the new shared (between FSX and P3D) directory structure so, basically, something older than June 2017, when P3D V4 was released.
If you ran an old installer AFTER you had all your FSDT/Flightbeam stuff migrated to the new folder, installing the Stand-Alone Addon Manager should be able to re-migrate the product installed in the old way, to the new folder structure, without uninstalling and reinstalling everything. And note that, installing a new full installer again, like GSX, will obtain the same effect.