Energy density (with respect to time) - J/m^2-s or equivalent.
As for "Who cares?" The GP for one. Me for two. Most people on Earth as well. The more land that is used up, the less you have for other purposes, be that for humans (agriculture, forestry, mining, grazing, etc) or natural habitat. It hits doubly that reservoirs target land defined most notably by the following characteristics:
1) Large river
2) Deep ravine/basin
3) Significant altitude change
In short, they often tend to be the areas most important to wildlife, often locally-unique habitats, as well as the most scenic areas within a given location - areas responsible as well for significant mobilization of sediment and oxygenation of water.
Solar, by contrast benefits most from environments full of endless identical flat wastelands. The more mundane and barren, the better.