While comprehensive coverage and collision coverage will protect your vehicle in the event of an accident or covered peril, regardless of fault, uninsured/underinsured motorist coverage is specifically designed to protect you from drivers without insurance or insufficient insurance.
If you get into an accident with an at-fault uninsured/underinsured driver, your insurance will cover your injuries or property damage.
Without this coverage, or without enough coverage, you’d most likely need to pursue the uninsured/underinsured driver in court or pay for everything out of pocket.