I believe one of the main benefits of demos is to expose that disconnect. You can ask people to think through how the software should work, and they may earnes

I believe one of the main benefits of demos is to expose that disconnect. You can ask people to think through how the software should work, and they may earnestly do their best, and they may give you clear answers about what they truly believe they want. But put the actual software in front of them and show it in operation or (better) have them try to use it, and suddenly a bunch of new insights will become crystal clear to them. In theory, people could reason through all this stuff, but it's not how most people's minds work. It's a natural tendency to avoid engaging your mind in what feels too hypothetical. Most people need to feel like they're in the actual situation before the mental juices really start flowing. The corollary is that the earlier you can do a demo, the less time you waste working toward requirements that are wrong. If all other things are equal, doing demos as early as possible should minimize that.