Now that we have a fairly good idea how Drupal 8 and data looks let's discuss what can MongoDB provide and why would you want to run it. In Drupal 8, every kind of data can be stored independently. I fully expect that people will indeed mix storages. For example, D8 by default runs a config query on every page to find the blocks to be displayed for the current theme. Again, by default, config entities are stored as serialized PHP arrays so the only way a query like that can run is to load every single block entity from the database and iterate over them.