Why do people with high blood pressure take water pills?

To reduce sodium i: Water pills remove salt and water from the body and this lowers thr blood pressure.

Water follows sodium: People with hypertension take diuretics like hydrochlorothiazide as a first line treatment. Diuretics cause the kidneys to excrete more sodium into the urine, water follows sodium, therefore there is less water in the blood vessels. Pressure against the arterial walls therefore decreases. This is also incidentally why a dash diet works. Decreased sodium in the diet=lower pressure.