Demos are a time to demonstrate all the features that have been completed in the previous iteration. Demos also highlight what's left to be done in addition to feedback on what we could've improved on.
What I particular like about demos is that it's showtime, time to demo hopefully all the great work that's been done. As a developer I sometimes want my code solutions to be noticed, but at the end of the day the end user usually doesn't care about the code, they care about their interaction with the software. Demos are a good reminder to me about what I'm ultimately aiming for, features to present to the client
Demos are so important that I feel that a rehearsal is required, similar to presentations, in fact I would say a demo is very similar to a presentation however with much more back and forth interaction with your audience(client).
This is something I'm trying to be mindful all the time during the week, how will I present the feature I'm working on, how many stories will I have completed in time for demo time. Without completed stories, demos can be very unproductive.