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.
Encode you message as the second letter in ever word. “Coded message” could be: “actually, your odd text added small fears as ester calls, egg deficiencies…”
In prison, you’d have plenty of time to choose more natural sounding words.