I see them as the "serving suggestions" or "other accessories sold separately" of video games.
Games appeal to something child-like in all of us. The "play" instinct. And companies know that, so they construct ads that appeal to us on that level. Go to the toy section of your local superstore, and look at the boxes and displays. Watch toy commercials.
The action figures are shown standing on their own, talking, all kinds of stuff... and the disclaimers trail along the bottom, "Toys cannot stand on their own," or "Does not talk," or "Other accessories sold separately." But for a child, the damage is done. They've SEEN it happen, so the belief is planted that this is how it is.
And even when the child KNOWS better, it plays on the subconscious. "Wouldn't it be super cool IF the toy could do that?" easily morphs into "Isn't this toy super cool?" without having to make any changes to the toy.
Fully-rendered trailers are there to show off elements that aren't in the game. They show detail the game will inevitably lack. But they serve to create a "serving suggestion" in your mind of how cool the game FEELS (or "could" feel)... whether or not any of that happens in the game itself.
Companies should universally be required to put "NOT ACTUAL GAMEPLAY FOOTAGE" on every frame. Otherwise, it'd be like movie studios using better actors in the trailer, and than D-listers when you go see the actual movie.