It adds a layer of obscurity, but not real security. If somebody is looking, neither sender or receiver can detect it or know if their ciphertext was intercepted. Depending on the methods used, the cipertext might not be immediately crackable with currently known algorithms and resources. However, it can be archived and broken at a later date, or by an actor who has access to algorithms/resources that aren't currently public.

Covert transmission is security. Think of a spy or North Korean dissident, mere detection of a transmission means compromise; Eve will extract the plain text using the trusty $10 wrench.

harvest-now-decrypt-later attacks aren't much of a concern for modern symmetric cryptography. heck, even known-broken ciphers like rc4 aren't easy to break in a non-interactive setting with modest ciphertext sizes and no key reuse.