There are two reasons you should keep your Bluetooth off when you are not using it. Bluetooth uses up your battery life, so turn it off whenever possible to conserve your battery. Turning Bluetooth off when not using it also minimizes your risk of becoming a victim of hacking via security vulnerabilities. Attacks like BlueBorne are becoming more common. BlueBorne could allow any device with Bluetooth turned on to be attacked. The best way to stay safe is to turn off Bluetooth when not using it.