Because they are going to be the Perth team when the time comes. They are preparing the way for Perth and sponsors are willing to be on board - it is quite obvious.
Except that sinking millions of dollars into an S.G. Ball team doesn't bring the NRL any closer to their goal of a Perth NRL team, it's just an expensive tangent from there actual goal.
I'm not privy to the decision making behind it but there could be a number of reasons; grooming a generation of players, sorting out any logistical issues without having to go through those teething problems in the top flight, giving the people of Perth hope / indication that a moves are afoot to bring them back to the NRL....
That is a monumentally f**king bad idea!
If anything changes in the proceeding decade and the NRL decide to go another direction with expansion then they've just set up the people of Perth for a big let down, and those let downs can have major impacts on the sport in the region, namely they lose trust in the organisation that's just promised them the world then let them down.
Like I said, if the plan wasn't for them to be the Perth NRL team, they would bother existing.
Because the NRL had nothing to do with their creation, and only funds them indirectly through grants, and the S.G. Ball teams existence is adjacent to the bid but not really a part of it.
The Pirates were a re-brand of the WARL's teams from the Reds, the bid, S.G. Ball team, and other representative teams, were all created prior to the NRL buying out the WARL, i.e. before the NRL had any direct influence over the WARL.
The S.G. Ball team was as created with the intention to create a more direct pathway to professionalism for juniors from WA, mainly with the goal of making it so that juniors that have a goal of making the NRL don't have to travel to the Eastern states at the age of 13-14 to have any chance of that happening. It also probably had a lot to do with the fact that the WARL had the means to create the S.G. Ball team and it is a good asset to have if you can afford it.
The bid was created by the WARL to, well, bid for an NRL license, because they want an NRL team in Perth and at the time nobody was actively bidding for a license in Perth, so they figured if not us then who, but in the last 8 years a lot has changed and it looks like there're going to be at least a couple of bids from Perth when the NRL chooses to expand (unless the bids join forces, which has be talked about a few times), so the Pirates aren't the only kids on the block any more and it's possible that they wont have the best bid on the block any more either.
Again to make it as clear as possible the bid and S.G. Ball team was created prior to the NRL buying out the WARL, so the Pirates S.G. Ball team can't be part of a grand conspiracy by the NRL to create a club in the NRL that they indirectly own and operate, because A. they had no influence over their creation, and B. if they wanted to create NRL clubs that they own and operate they don't need a conspiracy, they could do it tomorrow and nobody could stop them even if they wanted to stop them.