Initialize an already existing primary server

What's the correct way to initialize a an already existing primary server?

The [install page](https://pg-auto-failover.readthedocs.io/en/main/install.html#avoiding-the-default-postgres-service) suggests to avoid automated creation of a Postgres data directory.

The [pg_autoctl create page](https://pg-auto-failover.readthedocs.io/en/main/ref/pg_autoctl_create_postgres.html#description) explains how to initialize an already existing primary server with existing pgdata directory.

Do we have to change something in the existing pgdata directory? If not, why is the step to avoid automated creating of the Postgres data directory suggested?
Is it possible to use pg_restore and similar tools with a primary managed by pg_autoctl?