Most people who grow in dirt don't care about the water, but if you are doing DWC or HYDRO you do want to know what's is or is not in your water. Tap water can be anywhere from 100 to 400 ppm and well water even higher. If you target PPM is 800 and your water is 300 than you plants are not getting what they need, so with RO water the PPM is 0 to 20 ppm and Ph is 7.0 so when mixing food you know when your at 800 ppm your right on. The difference can be as much as 40% less yield.
The issue with chlorine and cloraminds in tap water is that they kill beneficial organisms in soil, so it won't kill your plant but it will kill your microbe army, and they help grow your plants.