Leadership is important because of what it brings to the group. Leaders provide examples, direction, mentoring, modeling, spiritual guidance, teaching. Leaders hold us accountable, provide directions that encourage unity, guard and protect the church, answer critics, and shepherd. Regarding elders, they are feeders, shepherds, pastors, elders, decision makers, overseers, overlookers, coordinators, organizers. They represent Christ's authority.
Leaders are important because they establish the atmosphere in a church.
Consider the example of David's mighty men, what a tribute to leadership. Think of churches you have known. The answer is often in the leadership. Realistically the problem is not only the preacher, but the entire leadership. The preacher may contribute to the problem or to solutions to the problem, but the problem is seldom exclusively the preacher.