An Audit Log contains an entry for each time any field throughout a database is modified, including when and who made the change and what the content of the field was both before and after the change. An Audit Log also contains an entry for each record deleted from any table specifying when and who deleted the record.
As for why this is useful, in many environments, either by corporate security policy or by statute or regulation, an authoritative and immutable log is required of all changes to all data elements in one or more tables. Even in environments where this not mandated, keeping such a log using fmDataGuard provides a means to offer much greater protection of the database against crashes and undesirable modifications. fmDataGuard offers both Rollback and Roll Forward functions that can operate on the log to reverse unwanted changes or update a backup to within seconds of a crash.