God said that everything must keep pace with the times, such as fairness in the nine realms. Therefore, there is a heaven complaint agency that can communicate with the nine realms and resolve disputes between the nine realms.
The Enterprise spacecraft, under the name of Heaven Complaints Bureau, travels through the nine realms and handles all disputes on behalf of God.
The complaint agency needs representatives from the nine realms to be fair, just and open, so Kirk came to the Enterprise as a representative from the human realm.
As a complaint handler, Kirk follows the Enterprise through the nine realms to handle various complaints.