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.
To help you figure out if you should purchase collision coverage, you should estimate the approximate value of your vehicle. While there are a number of online resources that can help with this, including Kelley Blue Book, we recommend you speak with a State Farm® agent. In addition to helping you determine the value of your vehicle, our agents can tell you how much extra you'd pay to add collision coverage.