How are foreign key indexes helpful?
This will be non-specific to oracle, but in general foreign key indexes are most useful because they speed up JOINs.

If you have a foreign key field, it is extremely likely you will use it as a JOIN key as well - that's the whole point of an FK, to make a direct association between one field in tablea to another field in tableb.

Having this field indexed will speed up any queries that use that field in either the WHERE filter or in the JOIN criteria.