Seconded, they apparently increase sales which isn’t surprising. Would you buy a book from an author you don’t know with nothing but a short blurb to go on? (I realise there’s a few where you’d know the author, but there’s a lot of authors without conventional published books and just because you can write good normal fiction, doesn’t always mean that your choicescript books will be the same.)
I’m also interested where you got that statement from as well. I’m personally far more likely to buy something if I have a demo or trial to see if I like the game I’m going to buy. If I can’t access a demo or return it within a time frame (like with steam) I’m far, far less likely to give games a chance I’m not sure of, so it gets more sales from me.
The only main downside to demos that I’m aware of is it opens you up to negative reviews because the game isn’t free.