For the lolz basically.
But there's some logistical issues with it. Given it's free tickets, who's going to pay for the public transport services? AFL? Or will the government step in and cover it for everyone? If it's on a Sunday, they might do that and just call it a free public transport day for everyone. The option is to make everyone pay for it and install about 100 temporary smartrider scanners at stadium station. What do you reckon an AFLW game would get anyway if it's free? 30k? I'd certainly go along to check out the stadium unless I had something important on.
Other benefit is its a test run for a cheap event so people will probably spend a lot on food and booze and all those bars and kiosks get put thru their paces. Docklands had problems the first few games and had bad PR as a result, including a bad flooding event.
Adelaide Oval for the Test Match in December 2013 before the March 2014 full opening, had he Western stand and new Southern stand available in full, about 3/4 of the 7,000 ground level seats of the 19,000 seat eastern stand and all the open northern end seats and Hill available . Between 34,000 to 38,000 turned up on each of the first 4 days and about 17,000 on the last day for 3 hours before the Aussies won the Test. I think it was the second largest Test crowd ever after the 1930-31 Bodyline Test when 50,000 attended a couple of days of the five days, most people standing up all day in suits.
So the SMA put a lot of the stadium thru its paces, found teething issues and then ramped up staff, security, police and road monitoring costs etc for the opening Showdown to avoid the bad PR of Docklands back in the first dozen or so games in 2000 when it opened. They admitted they over killed on costs but didn't want a repeated of Docklands and cut costs over the next few games.