What is the purpose of 'uninsured/underinsured motorist' (UM/UIM) coverage?

Uninsured/underinsured motorist coverage protects the insured (and often passengers) when they are injured by an at-fault driver who either has no liability insurance (uninsured) or whose limits are too low to cover the full damages (underinsured). In those cases, the insured's own UM/UIM coverage steps in to pay for their injuries (and in some states, property damage) up to its limits. This fills the gap left by an at-fault party who cannot pay. Requirements and specifics vary by state. Understanding UM/UIM as protection against inadequately insured at-fault drivers is standard auto-coverage content on the casualty exam.