1.9. How Dovecot processes configuration files
The dovecot package provides the main configuration file /etc/dovecot/dovecot.conf and multiple configuration files in the /etc/dovecot/conf.d/ directory. Dovecot combines the files to build the configuration when you start the service.
The main benefit of multiple config files is to group settings and increase readability. If you prefer a single configuration file, you can instead maintain all settings in /etc/dovecot/dovecot.conf and remove all include and include_try statements from that file.