Some teams don't do demos at the end of their iterations. Many of the teams who don't do demos also have trouble finishing all the stories they committed to at the beginning of the iteration. They continue, iteration to iteration, not always finishing, not getting to releaseable at the end of the iteration. And, sometimes, these teams don't do retrospectives because they are not done.
There's significant value in a demo at the end of the iteration.
- The demo shows the team what they have done and not done in the iteration.
- The demo shows the product owner/customer what they have done and not done.
- The demo acts a a milestone–the team has to stop what they are doing to show the demo. They can't keep going without doing a demo.
If you're not demoing at the end of an iteration, reconsider. Use the demo to get feedback, record your velocity, and see if you are done enough with this project for now, or if you really need to continue working off this backlog.