If you are storing your private key on a device in an environment connected to the Internet, such as a PC or smartphone, you should consider the situation in which an attacker can intrude through the Internet. For example, theft can occur through malicious code hidden in files downloaded through means such as e-mail. In many cases, you will not be able to easily determine if the theft has occurred, unless you are an expert. Therefore, in order to safely manage the private key from hacker attacks, it is safe to store it in a separate place disconnected from the Internet.