Hey,
it seems to me that the 10.2.0 release notes which have found in Server 10.2.0 Release are a little bit sparse on this topic:
By default, the “apps-external” directory is included in config.php during installation. For new installations, there will be two apps directories so that the bundled apps are distinguishable from the apps that were installed or updated by the administrator. Existing installations will not change but, generally, this separation is recommended in all scenarios, as it makes upgrading easier and less error-prone.
If you dislike this functionality or it doesn’t match your deployment i think its probably the best to request more information / documentation from the development and/or documentation team.
Maybe you can create a new issue in either one or both of the following issue trackers to contact the development and/or documentation team:
I think those teams have the best knowledge to handle your concerns or questions.