A goal of Fedora was to have a "snapshottable" /usr that includes as much as possible (all?) of the generic OS files. That is, multiple machines running the same OS can have a shared /usr, and everything machine-specific in the other dirs.
> Myth #11: Instead of merging / into /usr it would make a lot more sense to merge /usr into /.
> Fact: This would make the separation between vendor-supplied OS resources and machine-specific even worse, thus making OS snapshots and network/container sharing of it much harder and non-atomic, and clutter the root file system with a multitude of new directories.
See https://fedoraproject.org/wiki/Features/UsrMove#Why_don.E2.8...