As I see it (totally speculating) the following are the marketing justifications for the GS:
- Selling miles to "Partners" is a very profitable business for airlines (AC's Aeroplan is a case in point, it is a separate business from AC that seems to make a profit only selling miles). Therefore by giving DM members an added incentive to learn how to maximize mile earnings from partners US is likely to sell a lot more to partners (I certainly learned a lot of tricks to earn more miles that I will continue to use now that GS is over)
- Continuing from the prior point, promoting their partners will encourage companies to sign up as partners.
- It is frequently not efficient to have big chunks of miles with different programs, so once people get big GS bonuses they are likely to start crediting all their *A flight to US (and they make money on those miles as well, I assume)
- Once people start crediting all their *A miles to US, they are likely to start (or continue) actually flying US, all else being equal, it always makes the most sense to fly an airline that you have elite status with
- It makes a lot of noise and reminds people that US is a *A partner and therefore a serious consideration as a choice of a primary airline