You wouldn’t just leave the keys to your car lying on the front seat in plain site when you leave the vehicle in the parking lot, would you? Yet, many people go to the time and trouble to use encryption to protect their data and then choose to store the decryption keys on the local machine where the encrypted data is stored. If an attacker gets access to the system and finds the key, he can decrypt the data.
For best security, you should export the key to a floppy disk or removable flash memory drive. This is called “offline storage.” Make several copies and keep them in different locations, just as you have an extra set of car keys made that you keep in a secure place in case of a loss