The "Block System UI" feature blocks the "Recent Apps" screen that appears when you press the menu button in the navigation bar at the bottom of the device screen. In some devices and under specific conditions, the "Recent Apps" screen can be a path to bypass Mobile Fence, so it is recommended to enable this feature if possible. However, this is not an issue on all devices, and it will not be a major problem if your child does not exceed the set device usage time. Therefore, if you want to take proactive measures, you can enable this feature in advance, or you can enable it only if a problem occurs.