The base OS is a known unchanging set of bits. Squirt this datastream onto a storage volume and boot to it and you have a known-working system. Then you can futz around with all the self-contained packaged apps you want, and no worries about weird interactions fucking over your whole system.

It's not for me, but I kinda see the appeal.

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.

It's when you can't set the volume to 0% so that everyone around you has to hear how hard you're working.