The most important system-related files being protected from change by yourself and others.
Ensurance that your base installation is exactly the same as the one tested and used by its developers. And thus an (in-)direct quality control and maintenance by the very people that work on it.
As the base system is not changing beyond what is provided by the devs, installation of applications is relegated to flatpaks (see Flathub for the App Store).
Flatpak is a packaging format that doesn’t interact with the base system to install software; think of it like how applications are installed on your phone. With this, you can still install software you need without compromising changes to the base system.