Reasons to Use NPTv6

Prevents asymmetrical routing
—Asymmetric routing can occur if a Provider Independent address space (/48, for example) is advertised by multiple data centers to the global Internet. By using NPTv6, you can advertise more specific routes from regional firewalls, and the return traffic will arrive at the same firewall where the source IP address was translated by the translator.
Provides address independence
—You need not change the IPv6 prefixes used inside your local network if the global prefixes are changed (for example, by an ISP or as a result of merging organizations). Conversely, you can change the inside addresses at will without disrupting the addresses that are used to access services in the private network from the Internet. In either case, you update a NAT rule rather than reassign network addresses.
Translates ULAs for routing
—You can have Unique Local Addresses assigned within your private network, and have the firewall translate them to globally routable addresses. Thus, you have the convenience of private addressing and the functionality of translated, routable addresses.
Reduces exposure to IPv6 prefixes
—IPv6 prefixes are less exposed than if you didn’t translate network prefixes, however, NPTv6 is not a security measure. The interface identifier portion of each IPv6 address is not translated; it remains the same on each side of the firewall and visible to anyone who can see the packet header. Additionally, the prefixes are not secure; they can be determined by others.