Here, the main reason for the local fire service attending during an emergency, is to provide additional water.
Foam a plenty for a few minutes, then the airport tenders need water to mix, creating foam. Local fire service send pressurised water tenders to provide additional water supplies whilst the airport fire service replenishes locally. One must assume that this has been provided, but the terms of the aerodrome licence means that the local fire service need only to provide a certain local response, any additional is in excess of the CAA minima.
Additional staffing and liasion with the the local emergency services are also provided by the town service.