It's more for either monorepos or in my case, to fix packages that have bugs but that I can't fix upstream.
So for me, in my specific case, the freetype-py repo has a rather big issue with Unicode paths (it will crash the app if the path is in Unicode).
There's a PR but it hasn't and probably won't get merged for dubious reasons.
The easy choice, the one that actually is the most viable, is to pull the repo with the patch applied, temporarily add it to my ./vendored folder and just ideally change the requirements.txt with no further changes (or need to create a new pypi package). But it's basically impossible since I just can't use relative paths like that.
Again it's rather niche but that's just one of the many problems I keep encountering. packaging anything with CUDA is still far worse, for example.