How many people do you interact with at work during a typical week or month?
Most leaders I know don't count the relationships involved in their work. However, if they wrote a list of the people with whom they regularly interact, many would be able to go beyond a simple list and could tell you something about each person. Some information may be work-related (such as how each team member prefers to receive feedback), and other notes would be broader (such as which sports teams matter to the support staff and that a key vendor has a new grandchild).
Fundamentally, leadership involves relationships. Whether you're the leader of a small team, the manager of a large retail store, or the CEO of a global corporation, you conduct work through your relationships with others. To be effective in those relationships, leaders must understand the perspectives of the people with whom they are working. What leaders need is Empathy, one of the twelve Emotional and Social Intelligence Leadership Competencies.