Note, it's been around since JDK 1.0, so 1996, 28 years ago (!), though it's not widely used.
But it allows sandboxing of libraries you use. I was using it to make a faulty library that was calling System.exit(0) - yes, a library that was shutting down the entire process, closing the main app... - not do that and instead wrap the exit in a regular exception.
Python and especially Javascript are reinventing a lot of "enterprise" ideas that Java invented 20+ years ago. Maven groupIds are another example from the Java ecosystem (they're used to avoid top level package name squatting).
IMHO it's like the difference between basic research (mostly not very useful on its own but extremely important) and public health in medicine (the thing that actually makes a massive impact on the world that leverages the former but at a slower and more controlled pace). To stretch the analogy even more any doctor worth their salt will tell you that a treatment plan is only as good as the likelihood that the patient will stick to it. Other things like let's encrypt (https everywhere) and signal/whatsapp/telegram (strong privacy enabled by default) come to mind as implementing ideas on things that experts had been handwringing for decades that people "should" be doing that are now done so well that it feels almost silly not to do them the good way.
https://wiki.python.org/moin/SandboxedPython has some details
It would be cool if there was a universal access control platform, but I guess the best we're getting is Docker, because such a platform would commoditize OSes, which OS creators for sure don't want :-(