Hard to comment specifically, but if you're playing in the USA then I imagine NBA teams are big draw-cards. Playing an "Australian" team perhaps provides some interest, and presumably is significantly cheaper than 2x NBA teams.
End of the day, there are a lot of upsides for the NBL team, so they're presumably not looking to make money out of it.
It's not about proving an NBL team can match an NBA team. Nobody's going to believe that regardless of the result.
In marketing and advertising, businesses spend squillions purely on creating brand-awareness. So really this is about reminding the yanks that we do have a league, and what the level of quality is. We keep talking about viewing the NBL as a development pathway, and some players have had success in recent years, but it will be an ongoing battle to firm up that view.
It's also a great experience for the players, and potentially great exposure.