the aluminum (conductive) foil, when the cover is in place, makes contact with the conductive graphite coat of the cavity, and those together create what is called the
Faraday Cage, which shields the circuitry from external static electricity or RFI, if i'm not wrong.
that's what some guitar manufacturers do, as a coat of graphite would do the job with very low costs and time saving. but usually, in custom-made, or hand-made guitars you may find copper or aluminum foil dressing the whole cavity.
