Sculpting is necessary because the scanning process does not provide a mesh that is suitable for rendering or fabricating. No scanner can capture hair or eyes very well. And all scanners create some surface artifacts. ZBrush is a great tool for mesh retopology and digital sculpting. It also has useful tools for scaling and generating the STL files necessary for fabrication.
Large sculptures must be fabricated in multiple parts. ZBrush is used for cutting the sculpture and adding features necessary for holding the parts during CNC milling operations.
For more information: