Having uninsured/underinsured insurance means that if you are in an accident caused by someone else, and they do not have the insurance to cover the damage they cause, you can file a claim with your company to receive compensation. Depending on the amount of coverage you have, your insurance company can make up the difference. This can save you from complete financial ruin in the event of a costly accident.
It does not seem to fair to have to pay for something that is not your fault, and illegal to boot, but sometimes life is not always fair. You should evaluate your circumstances and decide if having this coverage is worth the cost. Usually, if you are buying other types of insurance as well, getting uninsured/underinsured motorist coverage does not add too terribly much to your premium. And, like other types of car insurance, the amount of money you expect the insurance company to pay out determines your premium.
Get "the whole package" of car insurance can seem very expensive. After all, there are different coverage options, you cannot just get "insurance" and expect it cover everything. You have to choose which types of coverage you want. But, if you are a good driver, and if you purchase other policies (home, life, and other insurance), you can receive special discounts that can lower your premium costs.
These factors combine to actually make a rather interesting insurance plan. You can tailor your coverage to match your needs. You can decide which types of coverage (beyond required liability) that you need. While you might not be able to control what another driver does, and whether or not he or she obeys the law, you can protect yourself. Having uninsured/underinsured motorist coverage is a good place to start.