You will water less often.
You make your nutrient solution in bigger quantities=greater accuracy.
You get big honking plants.
It's reusable. (if you disagree, just look at big honking plants
)
Hempy may be the least wasteful of water and nutes, if done properly. Watering the three big girls when dry uses about 1 3/4 ga eachl to get everything damp and the res overflowing a little bit. Everything gets taken up by the plant. I never throw water down the drain. Well, maybe a lil.
I buy RO from the supermarket in two 5 gallon jugs that i own. $.33/gal. I think I'm going to need another one . ..