For one thing, you might be the upstream developer of a library the program links against, and your stack frames might be hidden under frames from the program that didn’t save the frame pointer. Or if it’s a library that isn’t saving the frame pointer, vice versa.
But the more interesting use case is if you’re not the upstream developer of anything, but a skilled user who wants to get a view of your entire system and diagnose problems yourself. Personally I do that all the time, both in my spare time and at work. With respect to performance in particular I have more experience with the macOS tooling than the Linux tooling, but it’s analogous – with tools like Instruments and dtrace I can get a profile of any process I want or of the entire system, and I find that incredibly valuable. And that’s made possible in part by the stock macOS toolchain enabling frame pointers by default.
The NixOS case you linked sounds rather annoying, but turning on optimizations and -Werror and PIC by default is very different from just enabling frame pointers by default.