All of us rely on leaders in some way.  Like many animal species (humans are still animals after all) we form groups and somehow either through a natural or unnatural process (e. g. voting) someone assumes or is given the role (or perhaps roles in the case of multiple leaders).

Why do we need leaders?

Vision, mission, and  strategy are invaluable in all organizational situations.   From government, home, charitable to corporate and all types of organizations in between having a single point of reference to define, coordinate or facilitate the organizational road map  is only practical.

That is to say, multiple places where the buck stops has been proven experientially throughout history to be impractical. Consensus and collaberation are worthwhile ideals but ultimately even in collaberative high performing teams, the group looks to a single point.

What defines a leaders success?

Everyone has unique requirements of a leader.  This is why self awareness is such a critical skill for leaders.  Understanding what the people want, person by person is the first step.

Knowing the needs of others and facilitating their achievement is the foundational behaviour of great leaders.  Of course, there is more to it than just helping others achieve success, but also creating an understanding of what the objectives are.

Most simply put, leaders enable the achievement of success. Those that enable success the most seamlessly and where the results are most notable, are the best.