Just purchased a dozen of Easton Axis arrows and want to put brass inserts in them. What is the advantage of boosting up your FOC?

The primary reason that archers boost their FOC is to increase their overall arrow weight, thereby increasing kinetic energy, momentum, and penetration.  All things equal, a heavier arrow will provide more energy and penetrate better than a lighter arrow, which is why things like brass inserts and other weight systems have become increasingly popular in recent years.