It is KH, not GH that buffers the water against CO2 induced pH changes. GH is a measure of the ppm of calcium and magnesium in the water. Plants need both to grow well, so if you are uncertain about whether your tap water contains more than a trace of magnesium, GH booster will ensure that you have enough magnesium. And, you get some calcium as a bonus. Yes, calcium is more important, but most tap water contains some calcium, probably enough.