Miners, principally. And their attendant beaurocrats shuttling about from Perth to mines in central South Australia- their flights are often early-morning or late-evening, and so a place of rest at the airport would be most welcome.
And of course it will suck some tourist trade, it will be promoted that way since the Airport Corp. stands to gain significantly from premium services on site. But I still think a better (rail) link from the airport to the city is necessary- the current tram could be extended along North Tce, down West Tce and then out to the Airport via Sir Donald Bradman Drv. This will improve access and replace the truly inadequate bus 'service' that currently runs.
Of course, my preferred option is to build an actual train line, probably turning West south of Mile End station and running parallel to Richmond Rd right into the airport. It would stop at IKEA, the airport terminal/hotel and then terminate at Harbour Town, unless I could convince people to make it go to West- or Henley Beach and terminate there.