Unlike most game developers, who typically leave the selection and licensing of tracks for inclusion in games to the publisher, Rockstar makes a point of developing and licensing game soundtracks themselves. This can result in high visibility for songs that might not receive any attention from the mainstream market otherwise. In fact, according to Rockstar music supervisor Ivan Pavlovich Amazon had less than 40% of the songs from GTA IV available for sale before when they originally inked the deal to sell it.
While video game related promotions could certainly prove to be a useful marketing tool for both labels and artists, most game developers and publishers don't have the focus on music that Rockstar does, and certainly labels haven't shown any sign they've mastered the art of establishing new markets themselves. So the real question seems to be whether this is the beginning of a new trend, or just one more thing that sets Rockstar Games apart from their competition.