Why you cannot design professionally without it: Without the library, you would have to manually model every screw, pin, and plate from scratch. The library ensures that your designs are true to industry standards (DIN, JIS, ANSI), that they update parametrically with the mold size, and that your BOM (Bill of Materials) auto-populates with correct part numbers.
Once you have obtained the raw library folder (either via download or from the ISO), follow these steps precisely. Nx 12 Mold Wizard Library Download
Downloading the file is only half the battle. Correct installation is where most users fail. Why you cannot design professionally without it: Without