Underinsured coverage is probably the best insurance buy on the market. For example, without it you could be involved in an accident where another insured driver is entirely at fault, and still have problems. You could discover his/her insurance limits are too low to cover your injuries. It is at this point your underinsured coverage steps in and “increases” or “adds onto” the liability coverage (bodily injury) available for your claim.
NOTE: Most companies combine uninsured and underinsured coverage.