It might be helpful to think of the principal function of a group leader as facilitating problem solving and conflict resolution. Teams need a leader to see to it that their problems and conflicts get solved. 
One could make the case that a completely problem-free work group would not even need a leader—at least not very often. If a group could always function efficiently and productively so that its members always experienced a sense of achievement, group cohesion, high self-esteem and personal worth, obviously there would be little need for a supervisor.
Only when groups have problems and conflicts do they sorely need leaders. That is, when the members are having problems getting their needs met; when the group is causing the leader a problem because it is failing to attain the goals of the organization; or there are unresolved interpersonal conflicts that interfere with the group’s ability to work together collaboratively and productively.
Effective Leadership Requires Specific Relationship Skills
Dr. Thomas Gordon and the company he founded in 1962, have been shouting this from the roof tops—and approximately half a million leaders later, we’re still going (and shouting):
We know the specific communication and conflict resolution skills that a leader needs to become an effective human relations specialist—one that encourages cooperation, collaboration, trust, respect and productivity.
It is clearly evident that an effective leader must be both a “productivity specialist” (meeting the organization’s needs) and a “human relations specialist” (meeting members’ needs and facilitating conflict resolution). One without the other does not work.