A playable demo can change my mind (and I am sure a lot of other people too).

When you present your concept to other people, people would usually form completely different things in their head. An easy example is to tell a group of people to draw a tree. The trees will be different. Some draw a palm tree, other might draw a spruce tree, etc. Now this is just a word. Imagine doing this with a whole stack of papers that you call design document. The images that form in their head are completely different than yours.

You can provide images, screenshots, but the "feel" of the play can't be shared unless you have a playable demo.

Theres also the problem that ppl might not like a supposed good game, some ppl dont like final fantasy games, for example. If that situation happens, the args will never help, and ppl will never realize the problem isnt the idea, but ppl tastes.

Theres this snes game, Troddlers, its one hell of a good game, takes lot of time to beat, the puzzles are really good, the controls are really good, I love that game (much better than lemmings). But Im the only person in the world who likes that game.

Imagine if I have invented the game, how terrible would be to me present the idea, I would be like "so my ideas suck? Im lying to myself?". I always have that perspective in my mind, but its also easy to fall in the trap of using it as excuse for all shit ideas you might have.