Remember that proving liability is only part of what you need to do in a car accident claim. Even after it is apparent that someone else was responsible for the accident, you still must prove your damages and negotiate your claim. In other words, fault is just part of the equation. After that, you will still need to fight for the compensation that you deserve.
Your lawyer will handle the settlement negotiations with the insurance company. This is a task for someone with experience dealing with these companies and knowing how they operate. It requires an experienced professional who knows when the insurance company is trying to underpay your claim. The average person may not know what their claim is worth on their own without the help of an experienced professional.