Why was the fact that Enigma never encoded a letter as itself useful to code breakers?

The fact that the Enigma machine never encoded a letter as itself was a critical vulnerability that aided Allied code breakers at Bletchley Park during World War II.

This mechanical limitation provided a reliable rule that cryptanalysts, including Alan Turing, used to validate their work. When using the "crib" method—where code breakers guessed a portion of the plaintext (such as a weather report) and compared it against the intercepted ciphertext—they could immediately discard any potential key setting that resulted in a letter being mapped to itself. If a guess suggested that a letter in the plaintext was represented by the same letter in the ciphertext, the code breakers knew that the guess or the assumed position of the crib was incorrect. This rule significantly narrowed the search space for the Bombe, the electromechanical machine designed to find the correct daily settings, thereby accelerating the decryption process.1