Speed of development. It could take months for a PR to get into Lemmy core and then a new release.
Things that get into Lemmy core have to be well thought out and the core Devs have to want them in there.
Running custom code is a way to make changes without having to get their approval, and if it proves popular enough, then maybe they’ll implement it upstream.
I was pretty impressed with it the other day, it converted ~150 lines of Python to C pretty flawlessly. I then asked it to extend the program by adding a progress bar to the program and that segfaulted, but it was immediately able to discover the segfault and fix it when I mentioned. Probably would have taken me an hour or two to write myself and ChatGPT did it in 5 minutes.