They have a source and a destination address, but anyone who handles the packet can look at the message. Even if you think you have no secrets you wish to keep, you should still use encryption because of its potential for use in authentication. If someone has your PIN or password, it is the same as if he or she had your passport or credit card number your identity has been stolen. This identity can then be used in ways that inflict legal, social, or economic damage upon you. These differ in their use of secrets.
Symmetric ciphers use a single shared secret for encryption and decryption.
If one knows this secret, it is easy to transform plain text into ciphertext and back. Without knowledge of the secret, however, this process is very difficult. The advantage of symmetric ciphers is that they tend to be much faster than asymmetric ciphers. They also function in a simpler manner, like a lock and key. The encryption mechanism is the lock used to protect information.