The Adventure Works example shows us the use of this for the Employee Dimension which incorporates both aspects: you have a ragged hierarchy for employees as each manager might not necessarily have the same number of level of workers under him \ her. You also have managers which have sales attributed to them, not only to the leaf - level employees.
In my own SSAS project, I found I needed the parent child dimension to describe my organizational hierarchy. This stemmed mostly from the fact I had budget allocations for different levels of the organization (not necessarily the team level unit which is the lowest level). Using the parent child dimension also enabled me to delete "unnecessary" levels which were created in the MF tables but were unnecessary in my SQL tables (these were mostly business units with no name which only existed in the MF tables as the MF tables only allow for the usual rigid hierarchies).
Last but not least, parent child dimensions are the only kind of dimension that you can write-enable, so that may also be a consideration for implementing them.
The Parent Child Dimension will have at least 3 columns:
1. The parent key
2. The child key (the key for the member of the current level)
3. The child name (the name for the member of the current level)
Both the parent key column and the child key column must be of the same data type. For the highest level (in my case, the entire organization), we may give a parent key which either equals itself, a non existent value key or null.
You can add a Parent Child Dimension to your cube, using the dimension Wizard. You define the key column to the dimension as being the child key column with it's name being the child name column. In a later screen of the wizard, you check the square for "this dimension contains a parent - child relationship between attributes" and the wizard should recognize that the relevant column is the parent key column, giving you a preview for the result.
When you use a hierarchy with a parent child dimension, you can use the HideMemberIf property that specifies when members will be hidden in a ragged hierarchy. For instance, you can choose if to hide a member if its name is identical to that of its parent.
For more details about how to incorporate a parent child dimension in your Analysis Services project please refer to the excellent MSDN article on the subject.