One advantage would be that I could have core OS without modified files for years, because core files are read-only. I could whenever decide to remove applications and libraries and to have fresh OS without breaking the core system.
In addition to that, hashes of the core OS files of exact version could be compared to a known hashes of the version and tampering of read-only files could be detected (for example if someone boots live CD and modifies core files).
Or, hacked core files are replaced on each boot and no comparing of hashes is needed.