Useful and useless, useless is useful. What is a turtle shell?

Zhuangzi's life is no different from that of ordinary people, except that "there is a specialization in the art". Therefore, Zhuangzi's views on the details of life can also lead to a philosophical reflection and further study. "Useless" is also a reflection and inspiration of Zhuangzi's daily experience.

Useless use, that is, seemingly useless use, is the greatest use. This sentence seems to subvert the world's cognition of "useless materials". Is it useful or not?