IPv6 NAT is also useful because the /48 IP block I get from my ISP may change. I switched from DSL to Fiber and I got a different IPv6 block. But because if that, I also needed to update IPv6 addresses to my local DNS server (which needs to work across VLANs - so link-local address is not an option there). With NAT66 I could just assign a private address space to my LAN and have it only translate it externally to WAN.