Final answer:
Disabling the system UI can lead to a non-functional device by removing essential interface elements needed for interaction, recommended only for specific development or troubleshooting tasks.
Step-by-step explanation:
When you disable system UI on a device, you are turning off the user interface elements that allow you to interact with the core functions and settings of the device's operating system. It can make the phone unusable, as essential visual elements like the home screen, notification bar, and navigation buttons may stop working. It's not recommended to disable system UI unless you are aware of the implications and have a specific reason to do so, such as during development or troubleshooting with an alternate means of interaction like ADB (Android Debug Bridge).