NFC (Near Field Communication) is the channel when you scan your phone very close to the sensor (within 4 cm) and get your latest blood glucose reading. Bluetooth is used to connect your Libre 2 sensor to your smartphone (within several meters) so that the LibreLink app in your smartphone can provide alerts whenever your blood glucose goes too high or too low.
Hi/Lo alarm monitoring occurs with no intervention on your part, and can, for example, wake you from sleep if your blood glucose plunges, or alert an arborist when he is 20 ft up a tree with a chainsaw that his blood glucose is plummeting. Unfortunately, Libre 2 is a very poorly designed product, and the Bluetooth communication fails constantly ("Signal Loss Alarm"), a potentially lethal product deficiency under many circumstances. Abbott seem quite disinterested in correcting this lethal design flaw, despite postings from thousands of users on diabetes forums around the world.
If you are using a Libre 2 sensor, and have turned off Bluetooth on your smartphone, then you are missing the only unique benefit offered by Libre 2, namely continuous hi/lo monitoring. But since this essential feature does not work most of the time, its utility is zero or worse, as a user may not manually check regularly, confident that he will be alerted if his blood glucose is dropping.