postgres: what is the archive_command and when to use it
The archive_command is useful when:


You want to do point-in-time recovery from a base backup to some past state in case of disaster, like accidentally dropping a database.
You're using streaming replication without a replication slot. In this case if the replica gets behind by more than wal_keep_segments and hot_standby_feedback is off or the replica gets disconnected temporarily, the master will remove resources it needs and it'll break. If you have WAL archiving in use, the replica can fall back to using its restore_command to fetch WAL to catch up and recover. Otherwise you'd have to re-create it from a new pg_basebackup.