Comprehensive car insurance covers damages from an "act of God," or events that are not caused by a car driving into something else. An "act of God" can include things like damage from a heavy tree branch falling on your car. Since you have no control over when or why a tree branch would fall on your car, this kind of accident would be covered under your comprehensive policy.
Underinsured motorist coverage reimburses you, a member of your family, or a designated driver if one of you is hit by an uninsured driver or a driver who doesn’t have sufficient insurance to pay for your total loss. This coverage also offers protection in the event a covered driver is the victim of a hit-and-run or if, as a pedestrian, you are struck by an uninsured or underinsured motorist.
Policies typically use vague language when referring to acts of terrorism, but they are generally insured by the comprehensive portion of your policy. For example, if there is an act of terror and you need to make a claim on your car, that can only be made if you have comprehensive coverage. Since some circumstances are out of our control, comprehensive insurance is certainly important to have in your policy.
Collision insurance is a coverage that helps pay to repair or replace your car if it's damaged in an accident with another vehicle or object, such as a fence or a tree. If you're leasing or financing your car, collision coverage is typically required by the lender. If your car is paid off, collision is an optional coverage on your car insurance policy.