The dummy grid is where the cars get organised by the grid marshalls before going out onto the starter grid for the race.
It reminds me of when you take your car onto the Cook Strait ferry. You have to turn up at least an hour earlier so they can organise the cars in a grid formation at the wharf so they can confirm that they can fit you all on board the ship.
… but they make you bring your luggage on separately… like all your tyres:
It’s when fans who are allowed in the area can get a closer (within reason) look at the cars, and the photographers can take some good pre-raceshots. The V8s were popular - TVNZ filmed the cars coming from the dummy grid.