From the official Twig documentation: "Macros are comparable with functions in regular programming languages. They are useful to put often used HTML idioms into reusable elements to not repeat yourself."
In Drupal 8 a macro is used to create eg. the main navigation (see the menu.html.twig file).
The objective of this initiative is to create an Open Curriculum for new developers to come into Drupal and get trained in the various aspects of Drupal development. The curriculum is aimed at helping Drupal companies train new recruits in Drupal in a systematic and structured manner. The training is expected to happen through in-class sessions, self-learning, training exercises and finally on-the-job learning. The open curriculum is expected to be a framework that will allow the developers to build an understanding of the technology and then fill in details as they go through the process of training and working on projects. See more at https://groups.drupal.org/node/508323
If you want to control how group members create and interact with group content through other means than the group settings on the node edit form (or just allow admin users to change what group content is in), it can be useful to hide that setting. One way to do this is to create a custom module and use this code that hides the form for all users except those with role id 5 (replace HOOK with your module name):