Organizational anatomy is the identification and description of the structures that make up an organization.
Where human anatomy describes the human systems/organs, their purpose and how they fit together, organizational anatomy describes the organizational mechanisms (i.e. roles, responsibilities, processes, etc) their purpose and how they fit together.
Understanding how something fits together is essentional in order to be able to take it apart, diagnose what's wrong and put it properly back together.
In the context of people, medical students learn about human anatomy in order to be able to diagnose and treat illness.
In the arts, knowledge of human anatomy is used to be able to draw and sculpt people in a way which correctly reflects how the human body looks and operates.
You might say that a med students motivation for learning human anatomy is to be able to dissect and diagnose, whereas an art student's motivation is to be able to design and develop their model of the human form.
These anatomical motivations can be transferred to the leader and organizational anatomy.
The purpose of understanding organizational anatomy is for a leader to to be able to better: