The problem with water is nasties settle into it & thrive. When a vaccum accurs inside the fermenter,it can get sucked back into the fermenter. That's one way infections start. Many of us use starsan or cheap vodka that kill anything settling into them. I like the vodka myself. It won't effect the taste of the beer if it gets sucked back & nasties die in it.
I never use anything but water and have never contaminated a batch. I don't chill my fermenter when it's done which could get me a bit of suck back from the airlock. Mine only has gas escaping.