In religious leaders, people find spiritual leadership, which is one of the important factors that causes people to follow religions in the first place. These religious leaders, whether they are pastors, the Pope, or televangelists help give firm and clear interpretations of the Bible and religious teachings so that the followers have some answers on how to live their lives.
In terms of the good religious leaders do, they can be incredibly motivating, especially for social goods. When the Pope calls on followers to respond to the crisis in Haiti, a massive outpouring of aid from Catholic followers around the world takes place. The power of one person to motivate large numbers of people to address tragedies and social injustices is undoubtedly a good thing.