Not all land vehicles are covered under your policy. For example, if someone causes an accident with an ATV, then the vehicle can't have underinsured or uninsured motorist coverage.
When you're in an accident with a negligent driver who isn't insured or who doesn't have enough insurance coverage, then it can be difficult to get the money you want. With your own coverage, you, your passengers in some cases, and other drivers on your policy are all protected. Instead of having to fight with the other insurance carrier or turning to a civil lawsuit, you can simply file a claim with your own insurance provider.
Source: Colorado State Government, "Can I purchase un-insured or under-insured motorist coverage as a stand-alone policy?," accessed July 22, 2015