Oracle Database in read only mode would not let oracle applications operations work as normal application works include DML operations on FND tables (at least there are probably more than them).
If there was n option in Oracle Applications Database to run it as readonly but with exception to let normal/limited operations perform write operation.
This feature can help in ensuring that Archived/Pre-Upgrade Instances, which can be created for only Reporting/reference purpose as read only for users.
There could be multiple other scenarios like one could make a database readonly after applying a CPU/Rollup Patch (usually large patches) to ensure that for a time period post patch only reporting features are used till testing is completed.