They are pointing out that some locations are not a good place to grow specific things and that there is a lot of water wastage in doing so. Attempting to grow crops in the desert vs. in a temperate climate probably uses more water for the same amount of crops (unless they are desert plants, I guess). This is what's being pointed out. If I decide to grow tomatoes on the moon and then ship them back to Earth to be consumed, it's fair game for people to point out how much of a waste of resources that is vs. just growing them on Earth.