Two most common reasons for rental cars is after your primary vehicle gets into an accident or as a means of transportation during travelling. It is important to understand what your insurance policy covers and how to use them.
Transportation Expense Coverage - is used to pay for the rental to replace your primary vehicle while it is at the mechanics shop after an accident. Most insurance companies will have their preferred rental services because they have their paper work sync'd up and it will be much more convenient for the renter. Some policy covers the same class vehicle as your primary, others do cost per day. Ensure to know how much your policy covers and which vehicles you're allowed to rent so you don't see an extra bill after the repairs.
Collision Coverage - pays for damages to your vehicle after contact with another object.
Comprehensive Coverage - pays for non-collision related damage to your vehicle in events such as theft, glass breakage or fire. One-way vs. Two-way insurance policies - With one-way insurance, if you get into an accident that's your fault; your policy will only pay for the damages sustained by the other driver. With two-way, regardless of who's at fault, your policy will pay for the damages of your vehicle.
Insurance is very complicated; ensure you talk to your insurance professional before you purchase your insurance policy and what activates during an accident. Sometimes cheap isn't always better. For example you might not want to buy one-way insurance for your child's first vehicle.