What is the role of a diuretic?

A diuretic is used to help remove water from the body. Whether this is in order to lower blood pressure, relieve swelling, help alleviate congestive heart failure, or some other function, the excess water is removed, and that's what matters. Many people like myself, with high blood pressure, take a diuretic in addition to whatever other medication they take to decrease blood volume. I personally have questions about this method and some possible side effects/problems that I see, but that's another topic.