My initial thought was that they were inserted as a diversion to those who thought a simple frequency analysis would break what appeared to be a simple substitu

My initial thought was that they were inserted as a diversion to those who thought a simple frequency analysis would break what appeared to be a simple substitution cipher. Whereas in practice, the 'expected' substitutions yield gibberish, and the actual message is encoded using poly-alphabetic substitutions to throw off simple attacks.