In most circumstances, the car owner’s insurance will cover any driver borrowing it with permission. However, it may be a good idea to purchase your non-owner insurance if you borrow the same car frequently and drive it long distances.
Non-owner insurance can cover bodily injury and property damage in the event of an accident, but it does not cover vehicle damage or your own injuries (for that, you’d need collision and medical payments coverage). Generally, rates for non-owner insurance are less expensive than standard auto insurance costs.
Non-owner insurance kicks in if the limits of the owner’s policy are reached, or if you are explicitly excluded from their policy. It can sometimes pay for personal injury or medical expenses, but it depends on the policy.