Like, say, you are a revolutionary who’s imprisoned, but you are allowed to write letters to your family. How would you covertly send a message to your fellow revolutionaries that only they can decode? (Your family are on your side will pass the letter along)
Obviously, it cannot appear to contain any obvious ciphertext, since the prison guards would just rip up your letter and not send it.
Just use a contraband cellphone. That gives you access to powerful encryption.
This is the right answer. I hear they’re not hard to get in prisons.