Desalination does not intefere with rainwater harvesting. Rainwater harvesting is an entirely parallel enterprise, mostly involving accumulating very large bodies of freshwater.
Currently desalination is very energy intensive and so tends to do harm due the pollution generated from producing that energy.
These greenhouses desalinate water with a fraction of the energy requirements (pumping sea water inland takes energy, but nowhere near as much energy as you might imagine, as long as the inland end is not too far above sea level), while producing food, increasing the amount of moisture in the local area so greening that area of desert, and also increasing the amount of water available to the local municipal supply, at a rate that can be offset by the main farming business.
This is only control of water in the way that a pipe is control of water. It is continuous production of clean water in areas where that is often a problem.