This requirement is explained in the following section of the user manual.
If it needs to be explained better, let's improve that section.
The bottom line is that a part (level-0 section) can only contain the following two elements:
* part introduction
* level-1 sections (which themselves can have nested sections)
A part must have at least one level-1 section. If it doesn't, it's considered invalid (because it's considered invalid in DocBook).
-Dan