Flashback logs were introduced in Oracle Database 11g.
These optional files allow the database to be rolled backwards in a similar fashion to a redo log allowing a database to be rolled forward.
Although flashback is possible using data in the UNDO tablespace, such operations are limited to the available UNDO extents which are subject to the UNDO_RETENTION target and may be prematurely expired if active transactions are generating undo blocks.
In addition, flashback logs are required if a database is to be rolled forward through a reset logs operation using redo logs from a different database incarnation.
Flashback logs add up to a 20 percent performance penalty due to the additional I/O they generate.
Dell EMC recommends disabling flashback logs unless there is a compelling business case for their use. Most replication and disaster recovery use cases can be handled by array based technologies such as Recover Point.