The system (the os files to be precise) is only mutable by package manager for specific tasks like updating. It can break certain workflows if the user wants to change system files, because they can't.

Bonuses from that are security and reproducibility. You can be sure that whatever package you have will look and behave exactly the same as on another device with the same OS. Malware won't be able to mess around with your OS so trivially as it does on mutable distros.