Postgres: Purpose of archiving in master?
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.