The demo scene is meant to provide a unified place to test out how different object interact with each other, while keeping an eye on general performance. You can grab objects, interact with switches, levers, and buttons, and even do some climbing and combat.
Check out the script /Scripts/Scenes/Demo/DemoScript.cs for some additional code that is used in the demo scene.
The demo scene will be updated regularly with new features, so I wouldn't recommend saving your modifications to that scene. Instead, copy the prefabs over to your own scene, or simply save a copy of the scene to your own folder.