The Micro Site module allows you to set up a Drupal web factory, from a single Drupal 8 instance, to power a multitude of sites, of a different nature if necessary. Micro Site provides a new content entity that allows you to customize these different site types at will, using the APIs available on Drupal 8, and modify their behavior according to business needs.
Composer has become a must for relatively ambitious Drupal 8 projects. Even if it is still possible to initialize a Drupal 8 project with drush or simply by downloading a zip archive, these two methods can become limiting over time. Or at least not to facilitate the installation of new modules with dependencies on third-party libraries.
Another of the reasons I have encountered, why some Drupal 8 projects have not been initiated by Composer, is the lack of Composer support on some shared hosting, even so-called professional ones.
Drupal 8 is a tool designed to meet the needs of the most ambitious web projects. We hear a lot about the notions of headless, API first, decoupling, etc. that resolutely allow solid architectures for ambitious projects. But this does not mean that Drupal 8 no longer propels more traditional, and sometimes even much less ambitious sites: simple, small, and even large, websites, but for which we want to benefit from the modularity, flexibility and robustness of Drupal.
In some cases, it can be extremely interesting to be able to override a configuration dynamically. One of the first use cases immediately noticeable is in the case of a site factory with a set of shared and deployed features, and therefore identical configurations shared.