If you've got software that runs only in OS 9+ and that you still use, then installing OS 9 makes some sense. If not, then there's no good reason to have it on your hard drive.
When I went from Jaguar to Panther a few years ago, I had one favorite game that runs only in OS 9, but I couldn't justify installing it just for that one game. It would just be sitting there in the hard drive taking up space 99.9% of the time. So I've been without OS 9 since that time and haven't looked back.
While some people still use OS 9, it generally is considered somewhat of a dinosaur at this point.
I'm not aware of any software being written for OS 9 anymore.