Turning on the Bluetooth on your mobile device enables other Bluetooth-enabled devices to see your device and possibly make connections with it. It could allow an attacker to install malware through that connection, or secretly activate a microphone or camera to eavesdrop on you.
If you have an old computer you want to get rid of, it’s important to remove all your personal data first. When you think about it, it’s not just computers that contain personal information – devices such as smartphones, tablets, pen drives, and even game consoles contain personal data. Many people replace their mobile devices as often as every year, without securely removing data first.