Even if NAT itself had no security benefit (and it does have a little, even if weak), NAT coming into existance improved the average home network security by a LOT. In the earlier days of the internet, when address scarcity was a not yet a huge concern, ISPs provided multiple IP addresses to their customers. This allowed users with more than one machine on their home network to have nothing but a switch at the edge of their network (or more likely a hub, because they were cheaper at the time). Very, very few people had routers or dedicated hardware firewalls. The depletion of IP addresses, and the advent of NAT necessitated a level 3 device at the edge of most home networks. Having that device in place that was level 3 allowed device manufacturers to add firewall options as features to attract customers to their device, and since consumers now HAD to purchase the device anyway to make everything work, they happily picked options that also advertised extra security. Part of security is making it easy for the average consumer to make a good decision. Not everyone is a security concious individual that will buy, or find and install supplemental security software or hardware if they aren't nudged in that direction in some way.