Configuration is used in Drupal 8 for storing everything that has to be synchronized between the different environments (for example, moving from development to production). As such, it differs from the other types of data storage we have seen so far in that they were specific to one environment.
Another way of looking at configuration is by examining the role of a traditional site builder. They typically navigate the UI and configure the site to behave in a certain way—show this title on the home page, use this logo, show this type of content on the home page, and so on. As we mentioned, the result of their interactions materializes into configuration that the site builder expects would travel easily to the acceptance environment where it could be reviewed, and finally, to production.
Some configuration can actually be critical to the proper functioning of the application. Certain code might break without a parameter having a value it can use. For example, if there is no site-wide email address set, what email will the system use to send its automated mails to the user? For this reason, many of these configuration parameters come with sane defaults (upon installation). However, this also shows that configuration is a part of the application and just as important as the actual code is.