The word for turtle in Mandarin Chinese is a homophone for the word "to return" or "to bring in." According to a book by a Qing-dynasty scholar named Chu Shaosun, the tortoise will "bring in" wealth and the whole household will certainly prosper.
而國語龜與歸的讀音相同，清代學者褚少孫在《補龜策傳》中就說：「能得名龜的，財物歸之，家必大富。」