Why take low dosage of blood pressure medication 12.5 mg of hydrochloride if my blood pressure jumped around but was never high.

It depends: It may actually be for your kidneys, but not highly likely. More likely, it is to lower the "peaks" of your blood pressure, so they do not get as high. If you see #s >140 / >90, then your doc is likely doing this. If not, then you should ask your doc the rationale for the med since nothing is risk-free.

How high?: If your average BP is >135/85 (either number), your risk of stroke, heart attack, congestive heart failure, and kidney disease will be lowered by lowering your bp. 12.5 mg of hctz (hydrochlorothiazide) has been shown to be effective, reduce death from all causes, and has virtually no side effects for the vast of majority of people who take it. It's also dirt cheap!