The rest of the section is sparse on what you actually do with it. It says that it "has the potential to be a powerful tool" but not what you do with the power. It says it's about "enabling the fundamental capability of multiple isolated interpreters" but it's not clear what the capability brings you. The only thing with some details is the first sentence - "Running code in multiple interpreters provides a useful level of isolation within the same process." But what is the level? It sounds like it gives you isolation for most things but not all, which is about as useful as a face mask with a breathing hole cut out in it.
It'd help to see a concrete answer of something you can build with this that you can't build without it (or perhaps not as easily/performantly/reliably/etc.). The Ceph PR, where they do something similar themselves (https://github.com/ceph/ceph/pull/14971), gives a good answer: "Notably, with this change, it's possible to have more than one mgr module use cherrypy." (But it's still not clear to me why the mgr can't just use multiple Python interpreters... ceph-mgr is kind of an amalgam of various useful services for your Ceph cluster, and it's never been clear to me why it has to be "the" manager with some modules instead of various independent services you can turn off/on as you need.)