What it does is simplify your network administration, particularly for large high-traffic networks.
- Running dual stack means administering two seperate IP allocations for every network. Running IPv6 with NAT64 means running only one.
- Dual stack networks can be a pain to debug, it can cause a lot of confusion if one protocol is working, but the other is not.
- Assignment of users to NAT44s must be managed at the routing level. This is problematic for a couple of reasons, firstly it potentially means routing based on source rather than destination, which is not really what routers are designed to do. Secondly it means there is no way to gracefully move load around or "drain" a NAT for maintinance. Assignment of users to NAT64s on the other hand can be managed through DNS, which makes it much easier to gracefully move load around.
- If you are very large then you may not have enough private addresses to give every customer a unique one. So not only do you have to manage private V4, you have to manage overlapping private v4.