It's common for a TV show to introduce new viewers to its setting with a compilation of clips. Early in the season, clips are carefully selected to avoid upcoming plot twists while demonstrating the show's general method of entertainment. As the season progresses, anything from previous episodes is fair to use in this in-show trailer.
During the era of physical products, game publishers have relied heavily on release-week sales. Game marketing continues to reflect this focus on immediate sales. Like theater showings of movies, games are advertised before and around launch... with little effort made to refresh interest (until DLC is pitched).
Well, if gamers are only interested in newly released games, explain to me the rabid interest in backwards compatibility for Xbox 360 games or countless other examples of retro gaming. Explain why Microsoft or Amazon bother to have temporary sales on older games. If there is a viable market for games months and years after release, then imagine how much better the market would be if publishers bothered to advertise those games long after launch.
Like those TV show intros, post-launch trailers could incorporate clips of gameplay from happy customers. In addition to attracting new buyers, the potential for a gamer seeing his own clip in an official trailer would help to maintain interest in the game and sustain a market for DLC, merchandising, sequels, or spin-offs. Alternatively, contests are a cheap way to produce trailers because fans will do most of the work for free.