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.
You’d need to arrange the code ahead of time with whoever you were communicating with. I can imagine a few ways to do it, the number of letters in the words, the arrangement of the spaces between words, first letter of first word, second letter of second word, up through five words and start over, prearranged code phrases of course but nothing that would work with an unwitting recipient.