Why do people use the darknet?

To hide your identity:

   People may not want their data collected
   It's popular among civil liberties groups, journalists, and those who maintain online privacy.
   They don't want to be found (they may be criminals or want to engage in criminal activity)
   They want access to questionable content
   They want to use hidden services - although TOR itself is not hidden, the sites and users under it are hidden under layers of darknet encryption.

For criminal purposes:

   Black market trading
   Buying illegal goods (e.g., weapons/drugs)
   Participating in forums and media exchanges for pedophiles and terrorists