1. The whirlpool position on the filter valve is used to bypass the filter an recirculate the water. This could be used if there was an issue with the filter. It would allow you to still recirculate the water. Also, this feature can be used if you are doing stain treatment and want to bypass the filter.
2. High pH will impact the effectiveness of chlorine. It will also bother swimmers eyes and make them red. Low pH can cause the water to be very corrosive. It can damage the heater, pump, metal ladders, and the surface of the pool. Low pH will also bother eyes.