Postgres: Purpose of archiving in master? I am reading tutorials on how to setup streaming replication in Postgres from master to slave.

Some tutorials recommend setting up archiving in the master (and restoring in slave) . In the master, this needs to be set in the conf file:

wal_level = hot_standby
archive_mode = on
archive_command = 'some rsync command to rsync logfile segments from master to slave'


What is the purpose of this archiving? When is it not necessary when setting up streaming replication?
WAL archiving is useful when you're running streaming replication, because there's a limit to how much WAL the master will retain. 

If you don't archive WAL, and the replica gets s far behind that the master has discarded WAL it still needs, it cannot recover and must be replaced with a fresh base backup from the master.

It's also useful for PITR for disaster recovery purposes.