It is also being pitched as a way to constrain protocol types, which there is similarly no other way of writing.
Constrained protocol types and constrained opaque result types are both true extensions of the language; there's no way to get their effects without cumbersome workarounds. But this cuts two ways if you need to support earlier tools versions. On the one hand, you can't enable them for clients using new tools (by declaring a primary associated type list) without finding a way to avoid breaking clients using old tools (by suppressing the primary associated type list). On the other hand, you probably can't use them yourself at all, because you have no way of getting their effect in code that needs to work for clients using old tools.