 If you can get a little more onto the plant instead of having it blow away, more gets into the plant to protect it. In terms of water pH, this is about longevity of the fungicide and entry into the plant. By keeping the pH down to around 6.5 or less, your fungicide will hopefully not break down as quickly sitting in the tank. If the water pH is lower, that matches more closely to rainwater, which plant leaves love. You see the theme here. It's about getting just a little bit more product into the plant.