Overview
RFB is an AR who really wants to be a RF. During her Skill, she gains realtime furthest targeting and increases her own Damage and Accuracy.
Since the the enemy frontline can often be extremely tanky into the lategame, RFB manages to still have some use where other generic ARs start falling off, as she can pick off priority targets in the back while leaving the obnoxious frontline for your tank to deal with safely. The caveat to this is that she has no AP and doesn't have CDR tiles for HGs, though her ability to equip a PEQ can be quite nice for Night against evasive backline.
Her SPEQ-set just makes her more effective at her role, with a stronger first Skill activation and a strong Gray Zone SPEQ that's a Focus Chip without the RoF penalty.
Overall, RFB is niche/optional.