This cuts down on excessive water loss. The wider the brim, the better the shade. Kangaroo rats never have to drink a drop of water. By processing the hydrogen in the seeds they eat and the oxygen in the air, they produce their own metabolic water. Unfortunately, humans can't follow suit, so we must pack our own.
Water is the clear line between life and death in the desert.
Hiking in the summer requires at least two gallons a day. When backpacking overnight and making meals, three gallons will keep you comfortable. At seven pounds a gallon, that's a lot of weight. If you line all those water bottles on the floor, it looks like a preposterous amount. Even if you're acclimated to desert heat, military research has shown you cannot acclimate your body to using less water.