Why Enable Unified Audit Trail in 12c

In my opinion, the reason in short is that the unified audit systematically unify all required audit operations into a single, consolidated fashion.

According Oracle 12c documentation, you can see what UA is about.

The unified audit trail, which resides in a read-only table in the AUDSYS schema in the SYSAUX tablespace, makes this information available in a uniform format in the UNIFIED_AUDIT_TRAIL data dictionary view.

We see more points in the same document.

1. ... does not depend on the initialization parameters (once enabled) ...
2. ... consolidated view enables auditors to co-relate audit information from different components. ...
3. ... security of the audit trail is also improved by having it in single audit trail. ...
4. ... performance of auditing is improved. ...
5. ... named audit policies that enable you to audit the supported components. ...

For more information, you may refer the documentation:
Introduction to Auditing