Access to the Desert is unlocked after repairing the bus stop.
Sand Fish, needed for the Specialty Fish Bundle, can be caught here in a small pool of water in the North West part of the map.
The Skeleton Key can be used to enter Skull Dungeon, also located in the North West part of the map.
In the South West of the map Oasis can be located there, inside you'll meet Sandy and be able to buy items like Rhubarb Seeds, Starfruit Seeds and a variety of random items.
You can also find coconuts and Cactus Fruits laying on the ground around the area.