Most of the water they use in agricultural practices today in the desert are coming from wells. If they’re draining out these ground reservoirs to perform monoculture farming already might as well use that water for something more sustainable. Monoculture farming is highly resource and energy intensive.
It also rains in the desert intermittently. Typically in severe flooding. So storing the rainwater in the soil by slowing down the flow of water, through the use of landscaping and vegetation, allows the water to infiltrate into ground rejuvenating the reservoirs. It’s also part of the natural and sustainable cycle of permaculture. More water = more vegetation = more soil production = greater volume for root growth = more stored water when it rains again.