In some states uninsured motorist insurance is required; presumably you'd have minimum required coverage whilst in those states with any policy since they generally guarantee minimums for any state. Clearly if they're offering it as an optional extra coverage it isn't legally required otherwise.
Uninsured motorist insurance also covers hit and run collisions and parking lot damage where the offending party is not identified, or so I've been led to believe. Your insurance agent would of course be the best to say exactly what is covered.
