Why are mri's used for a person was in a car accident and had a head injury?

To see damage. Ct scan is good enough to see blood to proceed with surgery, but to see seizure focus, abnormal behaviour or neurological deficits correlation with MRI is very helpful, to assess the damage.
Better than CT. An MRI is a highly accurate radiological study which shows the structure of the brain and can demonstrate very small structural abnormalities which can occur after a traumatic brain injury. In this regard, it is more sensitive than a ct scan of the head.