Although Art is right that shit happens, sometimes it's serious shit and very often someone is responsible through an act of commission or omission. Insurance exists to minimize the financial risk to any one person or institution by spreading it out among all the policy holders. I once went to a concert at a church where a speaker toppled over and hurt someone seriously enough to require medical attention. Who was responsible, the church or the concert sponsor? I don't know, but I can see why the church would want the concert sponsor to carry liability insurance which covers them as well.