NAT was created for one reason only: because there weren't enough IPv4 addresses to go around.
Port mapping and connection tracking firewalls were invented in 1989,[1][2] while network translation was created in 1994. [3][4] The private address space was only reserved in 1996.[5] The Firewalls book was published in 1994 (which meant that it was being written in the 1992-3 timeframe).[6]