your absolutely right it sounds like a no brainer... and it is.. HOWEVER you wouldn't believe how difficult it can be at times for some growers that don't have a solid grasp on basic growing. my best and favorite example is a guy who was PH adjusting plain water with advanced nutrients PH down.. He was using almost one full teaspoon per 5 gallon bucket. For anyone who isn't familiar with advanced PH- or + its some of the strongest on the market and generally one drop will swing a full 1-1.5 points in a bucket of plain water. Anyway he was feeding that super acidic water for weeks after planting, the plants fried .. but after he contacted me we figured it out and had him change things he went from the first picture you see below to the second picture 14 days later to the final pic which was about 2 weeks into flower. The little 90% dead plants came back to a full hearty harvest for him.
Acid fed plants
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14 days later just plain water that was 6.8
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2 weeks into flower
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So yes its very simple as long as you know your waters ph .. just being in a safe PH range is all that matters theres people that have a 8.3 water PH which is way to high for soils to buffer and it has negative effects with uptake. in those cases you not only need to know you have a high water PH but you need to properly adjust it.