People may, or may not, choose to act interdependently. This creates both a challenge and an opportunity to lead. Leadership is the process of influence. Defined as such, every attempt we take to influence is an attempt to lead. As such, the need to think about power is also a critical aspect of assessing our own leadership.
Power is the capacity to influence.
No one is influenced without a reason. It is important to recognize that power is something other people give you. They also take it away. If someone thinks you have power - you do. If someone thinks you lack power they won't give it to you.