Consider the liquid diet your plants must live on - Think how it would be if you were forced to live on a strictly liquid diet. Just water and digestible liquid foods to keep you healthy. Because you are dependent on these liquids for life, you'll probably demand the best.
That's exactly the situation your hydroponic plants face every day. Without soil, they are totally dependent on the nutrients and water you provide. Skimp on either and your plants suffer.
Obviously, quality nutrients are essential. But what about water? Often growers don't give this essential ingredient a second thought.
You want to use the best quality water you can obtain. At the very least, it should be
filtered. But the best and purest water for hydroponics is obtained from a Reverse Osmosis system.