purpose of this parameter.log_archive_config

Hi,

Could someone explain me why is this parameter log_archive_config is introduced with practical example.

Purpose of this parameter (as per my reading).
1) dynamically add the physical standby database in case of RAC
2) Log transport will not be allowed to any database not on the list . 

But still I am not clear

1) what happens without this parameter ( How was it handled in case of Oracle 9i)
2) Purpose of introducing this parameter (with real time example)

Thanks.
Chakra