Auto insurance is financial protection, and not just for the investment you made when you bought your car. After a really serious accident, bills for damage and injuries can easily reach into hundreds of thousands of dollars. If you happen to cause such a wreck, the victims could sue you. In the worst case scenario, assets such as your savings and home could be seized.
Collision coverage is limited to the actual cash value of the vehicle, and requires a deductible, which is the amount you'll need to pay before receiving benefits. Higher deductibles lower your premium but increase the amount you must pay out of your own pocket if a loss occurs. Ask yourself how much you would be willing to pay on short notice in order to save on your premium, or talk to your agent.
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.