In my oponion the two recent Steam Demofests have been a breath of fresh air for the gaming world and might be the first good thing in a long time to happen in gaming. Since the early 2000s demos grew mainly out of fashion and have been replaced by Early Access™ and consumer friendly refund policies. If a game had a Demo it was often out of date and a subpar representation of a game. But with definitve events like Demofest, developers have the opportunity to show of their goods through free and up to date Demos without the fear of piracy or location constraints of gaming conventions. If some Indie Dev shows me that the game is 90% done and the release is with a reasonable time, I surley can spend 20$ to help polish out the last flaws before release. Indie developers are often ride or die with their games and could use the income as early as possible.

Ironically the last games I preorded where AAA games. Resident Evil 2 and 4 Remakes, both of which had extensive Demos. I played them and immediately thought "Yep, this is it, that's worth $60."

Demos are an effective marketing tool and can fully justify a preorder.