Symmetric Key Cryptography#A message exchange using a symmetric key requires the use of the same key by all parties involved for both encryption and decryption. This necessitates distributing the key to all participants.
When using symmetric encryption/decryption all parties must trust each other, because they can read each other's messages.
Physical distribution of symmetric keys is the safest because an unencrypted electronic transmission of a key would be vulnerable to theft. Physical key distribution would make symmetric keys hard to use in an environment with a large number of users. Even with Physical distribution, the keys are still vulnerable to theft.
However, often some application uses a clever combination of encryption schemes to distribute symmetric keys over the network for background authentication.