Why would anyone need IPv6 to be incapable of doing NAT?

To answer your question: Who knows? Perhaps you have a shitlord ISP that only provides you with a /128 (such as that one "cloud provider" whose name escapes me). [0] It's a nice tool to have in your toolbox, should you find that you need to use it.

[0] Yes, I'm aware that a "cloud provider" is not strictly an ISP. They are providing your VMs with access to the Internet, so I think the definition fits with only a little stretching-induced damage.