I'm not an engineer, and the article doesn't state it, but there are probably benefits to the technology. For one thing I imagine the heat-content and thus energy storage density must be rather high, while the storage material is probably quite cheap. Water would have (a lot) lesser density, and most electrical batteries are way more expensive.

Additionally the molten salt may be good at insulating itself or at least easy to insulate against heat losses cheaply.

And maybe it can be fueled by energy forms other than electricity.