Showing cool cinematics is a good way to get attention, like many have said, basic advertising. Of course, when done right, the trailer can represent gameplay, giving gamers a basic gist of what the game is about. Of course, it can be done wrong. Starcraft 2's a fine example, with alot of people unfamiliar with the series thinking it's an action game instead of an RPG. I've yet to meet anyone stupid enough to actually get to the point of buying the game thinking this, but the point still stands.
Personally, I see those kind of adverts as heads up to interesting games, then I'll do some research myself.
Personally, I see those kind of adverts as heads up to interesting games, then I'll do some research myself.
