Yeah this has been beaten to death a few times already, but it has to do with the water chemistry so it's worth revisiting.
Water can have a neutral pH and still have a lot of buffering power (called residual alkalinity) making it unsuitable for brewing pale beers. My water has a pH of 7.1 yet I still need to dilute it with RO water when I make pale beers such as blonde ales or cream ales since it has a lot of bicarbonates. Our local water source is a man-made lake which is essentially a giant hole dug in a huge slab of limestone. There is plenty of calcium and magnesium in the water which causes the pH to read neutral at room temp. However, if I start adding acid to the water (chemistry process called titration) the pH will not change until I have neutralized all of the carbonates. I know from experience I can neutralize the carbonates in 5 gallons of water by adding 2.5 mL of phosphoric acid. I do this to treat my sparge water when I make a dark beer with 100% tap water, and leave the mash water untreated. The mash has a good pH from the dark grains - in the 5.4 range according to test strips; however the buffering power of my water will cause the pH of the last runnings to rise to 6.2 or higher, which can extract tannins from the grain. Titrating my sparge water to neutralize the alkalinity keeps my last runnings at a pH of about 5.8, and as a few other folks have noticed, seems to increase the yield slightly as well.