This is how the majority of ipv6 is deployed where I live.

The router in a coffee shop gives you an ULA, and NATs everything to a single globally routable public ipv6 address.

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.

In any scenario where you want to do traffic steering at a network level. Managing multiple network upstreams (e.g. for network failover or load balancing) is a common example that is served well by numerous off-the-shelf routers with IPv4. That's an important feature that IPv6 cannot offer without using NPTv6 or NAT66.

It's conceivable that OSes could support some sort of traffic steering mechanism where the network distributes policy in some sort of dynamic way? But that also sounds fragile and you (i.e. the network operator) still have to cope with the long tail of devices that will never support such a mechanism.