Listener plays a very important role in the dataguard environment. Without the listener configured on the standby database, the primary database would not be able to communicate with the standby database.
Refer this on how to create a physical standby database in oracle 9i. Hope it helps !
Listener.ora file is very important in both the places.
1. If you do any switchover that time your standby database will be primary database. All the archive log files start sending from new primary database (prev'ly standby database) to new standby database (pre'ly primary database)
2. If you do any failover that time your standby database will be the production database.