Well, it's called a "proving ground", so it's a military training/testing area. Used to test new weaponry/ships, and the proficiency of the pilots/wings who take part in military exercises. Those who have proved to be proficient might get rank increases and/or special mission access? It would make sense for there to be a few such sites dotted about, in Empire space as well. Having one in the Sol system seems a bit sacrilegious, but then again, Salisbury Plain holds such a site and that's where Stonehenge is, so there is precedent.
The idea of adding more big new stuff when there's so much to do on the stuff that's already in the game does seem an odd way to manage dev resources, but perhaps they are working on the other stuff too. More details required before any judgement can be made, really.