Its a single momentary snapshot in time taken under less than ideal conditions on which a long term medication decision is based, often with attendant side effects. My understanding is that the proper way to chart for a BP problem is to take BP twice a day, once before lunch (after sunrise metabolic amplification affects are have waned) and again before right dinner time. These readings done daily for a week, and averaged are your real blood pressure readings. And then of course there are also the confounding effects of white-coat syndrome.[1]
So, not to downplay the absolute importance of detecting hypertension and treating it asap, but the way it gets done is both perfunctory and quite prone to false positives. This methodology sells extra meds for spurious reasons...