Can't install Mac OS onto your Macintosh?

Every Macintosh computer comes with its own version of the operating system out of the box.

Some people, for any reason, want to have different versions of the Mac OS on the same computer.

This is possible only if you install a later version of the OS.

For example. Your Mac came with Mac OS X (Panther) 10.3.7.
Well, on that Mac you can neither install Mac OS X 10.3.6 nor any previous versions.
But, you can install any version of Mac OS X that came out after 10.3.7 without any trouble.

See this technical document for reference.