Non immutable just means that your system isn't locked down to make sure you don't accidentally break something. Some people have a strong preference for it one way or another.

As to why it might help, every distro comes with a custom mix of software and tweaks. Bazzite is fedora based while Garuda is Arch based. Some things may work better or worse with less tweaking.

So if you are frustrated enough to consider something else and aren't committed to an immutable distro, it might be worth experimenting with.