This was added for crunchiness when eating ochazuke however later it was learned that it played a role as a drying agent to prevent ochazuke from becoming moist and it was accepted as the norm.
これは茶漬けを食べる際に歯ざわりの良い物として入れられていたが、後に茶漬けが湿気るのを防ぐ乾燥剤の役割を果たしていることが判り、標準化した。