It is an NRL promotion, not the Warriors. They have nothing to do with it except for Jim Doyle liaising between the NRL and ATEED.
It also involves ATEED (the Auckland Council) so there must be some evidence of tourists coming into town for the game. When I first looked to buy tickets pre season I was surprised at how many platinum seats had already been sold (my season platinum for example). So my guess is there are substantial bookings from the visiting clubs.
The double header came about in exchange for ATEED giving up their 5th year of the Nines