Simply put, the role of the schema scripts is to create a database structure supporting your module logic. For example, creating a table where our entities would persist their data. The role of the data scripts is to manage the data within existing tables, usually in the form of adding some sample data during module installation.
If we look a few steps back, we can notice how
data_version from the database match the
setup_version number from our
module.xml file. They all imply the same thing. If we were to now change the
setup_version number in our
module.xml file and run the
php bin/magento setup:upgrade console command again, our database
data_version would ...