I wanted to spin up a mirror locally to do simple caching for docker builds but the tooling was lacking, there was a way to do a direct mirror of pypi locally but no other way of adding custom indices
[1]: https://www.sonatype.com/products/sonatype-nexus-oss-downloa...
We simply set it up as a kind of "passthrough cache", so if it didn't have the package it fetched it from pypi, and stored it to be used the next time someone wanted to install the same package.
Apart from being nice to pypi, we also got a bit of a decrease in CI runtime, because it fetched packages from the local cache 99% of the time.