They usually show a trailer with a disclaimer. Usually the much better looking trailers are footage of the game's engine, but not actual gameplay. Although lately I haven't seen that many major downgrades. It's usually worse at the start of a generation. As for why developers make good looking trailers. The whole point of a trailer is to generate hype and attention. Showing raw footage of the current build doesn't create hype, but criticism. The public pretty much demands these trailers, otherwise they deem it bad.
Last edited by Qwark - on 24 March 2018Please excuse my (probally) poor grammar