Q: How would that even work?

A: The magic of Linux! By combining userfaultfd with process_vm_readv, any userspace application can obtain a copy-on-write mapping (with some limitations) of memory it never owned. All it needs is ptrace privileges, which is to say, having the same uid usually works.
Q: Still, what do you actually need it for?

A: Dynamic binary analysis and instrumentation of applications with built-in integrity checks. As far as I know process_vm_readv isn't even detectable if the agent process is more privileged than the examinee process—so you're free to manipulate your private copy of the application in the comfort of your own address space