Employment that couldn't be replicated at GLA (or EDI) without acting as a drain on the tax payer?
The Forth Bridge, Edinburgh trams and so on generate significantly more daily traffic than PIK could ever hope. HIAL airports offers air travel options to rural areas and airports that otherwise have no local connection to major airports and economic hubs, PIK on the hand operates a dozen or so routes to the Med that are replicated half an hour up the road.
Theres no case at all for Prestwick to be operating as a state owned commercial airport.