In an age where fresh, potable water is in short supply in many many parts of the world, a water-purification plant is a clear benefit to any area. It cleans pollution from water until it's drinkable again, and provides it back to the water system. If not the human-use system directly, then to the rivers and lakes that feed the human-use system. The lakes and rivers don't care where their water comes from, so long as it doesn't have noxious chemicals, bacteria-infested sewage, or anything like that.
However, people do not like having water-purification plants near them. Water purification plants often smell bad. They have stinky pollution, sewage, and bad smelling treatment chemicals. They might, depending on their pump technology, be loud. This causes the "Not In My Backyard" principle to block most water-purification construction.
Of course, being the maniac that I am, I'm thinking of ways to build it in ways that don't make people complain. I'm thinking, deep underground. The odor likely won't reach the surface, and the sound will be muted by the soil above. The only way you'd know it was there would be cheaper water bills.
Unfortunately, this would also mean greater energy use, as the purified water would have to be pumped back to the surface. That's a drag.