Chapter 7 Firewall for FortiOS 5.0 : Firewall concepts : NAT : Benefits of NAT : Security Enhancements
  
Security Enhancements
One of the side benefits of the process of NAT is an improvement in security. Individual computers are harder to target from the outside and if port forwarding is being used computers on the inside of a firewall are less likely to have unmonitored open ports accessible from the Internet.