Next Big Future carries a good writeup: Sand that blocks water access, providing a stable water table which also seals away salt carried further down into the sand.
The ramifications for this are simply stunning. This won’t do you any good in an area like much of the US southwest, where the sand is merely blown rock. But for significant chunks, and for putting around areas which already tend to be ponds and lakes when there’s not a drought, this would be amazingly helpful. The capacity to reclaim the Sahara also comes to mind….which would be a blow for world peace, as thousands of muslim goatherds currently are constantly pushing south on the fringes of an ever-expanding desert, into animist zones, helping to create cultural conflict. Hydrophobic sand would go a long way towards helping to relieve that ongoing catastrophe.