If that's not an EMI shield, I might go back to school...

It's most probably there to shield either the amplifier circuitry, or trying to prevent the amp from emitting crap all over the place.
Why separate PCB? Because they did penny pinching and didn't use 4-layer boards int eh main PCB and couldn't integrate the shielding to the main PCB so it had to be separate.
Metal cans are expensive, metal enclosures are expensive... so that was their solution.