Not just for sand and snow, but most off-roaders will air-down their tires as soon as they leave the pavement. The lower the tire pressure, the longer the footprint made by your tire, providing more traction. It also allows the tire to conform to obstacles such as rocks and downed trees, again giving better traction. But if you go too low you risk spinning separating the bead or causing damage to the sidewall from pinching the tire between the wheel and the ground.

On normal-profile tires (65-80 profile, as in 225/70-R16) it is generally safe to go down to 15psi, which should double the size of your tire's footprint. Just keep your speed low, generally under 15MPH. Always air back up before driving on pavement (though I have driven as fast as 45MPH at 15psi for short blacktop segments between trails, but not generally recommended).

Don't try this trick if you have very low-profile tires; you will damage the tire.