Why did gradual replacement of Japanese Red machines with Purple machines help cryptanalysts?

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A Vigenere cipher table displays the alphabet in both the top row and left column, with rows shifted to create a grid for encryption and decryption. Highlighted letters illustrate how to use the table. An explanation and example using the words cipher, key, and plaintext appears to the right. — cryptology In encrypting plaintext, the cipher letter is found at the intersection of the column headed by the plaintext letter and the row indexed by the key letter. To decrypt ciphertext, the plaintext letter is found at the head of the column determined by the intersection of the diagonal containing the cipher letter and the row containing the key letter.

The gradual replacement of the Japanese Red machines with Purple machines aided cryptanalysts because it provided an enormous number of "cribs"—known or suspected plaintext-ciphertext pairs—between the two systems. By comparing the output of the older, already compromised Red machines with the new Purple system, cryptanalysts were able to leverage their existing knowledge to break the more complex Purple ciphers.1

Furthermore, the Japanese attempted to simplify the key distribution process by generating keys systematically. This flaw, combined with the transition period, allowed U.S. cryptanalysts to not only decipher the Purple messages but eventually to anticipate the keys several days in advance. These efforts were part of the broader Allied intelligence project known as Magic, which proved instrumental in the conduct of World War II.1