To drive desalinization plants and solve the water crisis in the Southwest.
While desalination is a great use of excess power, this is not an easy thing to do because the places where the water is needed are inland. Obviously it doesn't make sense to pump desalinated water 180 miles uphill from the Gulf of California to Phoenix, what you really want to do is to use desalinated water at the places nearer the coast so they can stop relying on the river water that comes from the mountain west, so the southwest can use more of it (and so the mountain west can keep more of it for our own use). But while you could get some benefit from getting the coastal cities using desalinated water, their use actually isn't that significant. The bulk of the water goes to California farmlands, and those are in a belt 70-100 miles from the coasts, and there are mountains in between. Not terribly tall ones, but enough to make pumping the water challenging.
None of this means what you say isn't a good idea, but it does mean that a lot of infrastructure has to be built to make it work. Big coastal desalination plants, big pipelines from those plants, fed by big pumps, and either additional reservoirs or perhaps large tanks in the mountains to buffer the water supply -- though only after peak supply rises to the point that it exceeds demand. Heh. That's exactly the same situation as with intermittent, renewable power, just shifted to water. Water is a lot easier to store, of course, but you still have to build the infrastructure to store it.
So, this is a good idea, but it's an idea that will take years, probably a decade, to realize... and we have excess power now. Of course, starting by tackling the easier problem of using desalinated water in the coastal cities while the infrastructure is built out and scaled up makes sense.