FCS created the Fusion because of a need for a high quality, easy to install system, that can be installed under the fiberglass (as preferred by boards using new technologies such as EPS). This also provides the consumer a higher performing fin system that fits the worlds largest range of premium fins.
The insert provides a base of high density foam. This helps disperse any forces on the plug. It provides a solid base so the plug does not rotate or sink into the foam. This is especially evident in EPS blanks of low densities.