Why is Nanjing paper-cutting called one of the three treasures of Nanjing?

Nanjing paper-cutting belongs to the Jiangnan paper-cutting line. The characteristics of Nanjing paper-cutting are flowers within flowers, topics within topics, roughness within fineness, and subtlety within subtlety. Now Nanjing paper-cutting is both a national intangible cultural heritage project and a United Nations intangible cultural heritage project. The representative figure, Master Zhang Fanglin, is the second generation successor of "Jinling Shen Zhang", a national intangible cultural heritage inheritor of Nanjing paper-cutting, and a researcher-level arts and crafts artist.