Uninsured motorist coverage is part of nearly every auto policy in Missouri and Illinois. It steps in when the at-fault driver has no liability insurance. Either because they were uninsured at the time of the crash or because their identity is unknown (a hit-and-run, in some cases).
UM benefits come from your own policy, but the insurer treats the claim adversarially. You're effectively in a contract dispute with your own carrier over what they owe you. Most UM cases involve negotiating with your insurer the way you would a third-party insurer.
Both states require auto insurers to offer UM coverage, but the limits and stacking rules differ. Stacking. Combining the UM limits across multiple vehicles on the same policy. Is more available in Missouri than in Illinois.