Journal standby state minimizes replication impact by providing the benefits of an active journal without writing the journal entries to disk. As such, journal standby state is particularly helpful in saving disk space in environments that do not rely on journal entries for other purposes. Moreover, If you are journaling on apply, journal standby state can provide a performance improvement on the apply session.
Generally, using journal standby state increases switch times. However, if your data group is configured to not journal on target (JRNTGT(*NO)) and you switch, changing the data group to journal on target (JRNTGT(*YES)) along with using journal standby state will improve switch times.
You can start or stop journaling while the journal standby state is enabled. However, commitment control cannot be used for files that are journaled to any journal in standby state. Some referential constraints cannot be used when the journal is in standby state. When journal standby state is not an option because of these restrictions, journal caching can be used as an alternative. See Journal caching.