Create a refresh group if you have a group of MV's that you either want/need to refresh together. A good reason is refresh timing (daily, weekly, monthly, etc.).
I hate to say it, but the MV docs are perty good at tahiti.oracle.com
One major advantage of refresh groups is that they allow an option for complete refreshes to begin with a delete, rather than a truncate. In the event of an error during the refresh process, you can issue a rollback to recover the previously stored data set -- with a truncate, of course, you can't do this.
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