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Driving on soft sand
Why it is necessary to deflate tires in soft sand (or snow)
Most modern 4x4 vehicles have tires optimized for highway travel. They are as narrow as possible to reduce noise and friction for improved fuel economy.
Narrow tires make cars sink deep in soft surfaces like snow and sand. When sunken in, the front tires have constantly a small berm of sand in front of them. While driving they push this berm without ever being able to climb it. That produces a lot of extra resistance. More resistance requires more torque to keep the vehicle moving. But traction is not good on sand. The sand particles are not stable and tires tend to dig in deep - only creating more resistance. You'll be stuck in no time.
By airing down your tires, their footprint increases and they don't sink in as deep. That translates to less resistance and a lesser chance to dig in. Translates to a lesser chance to get stuck.
How much should you air down?
A a standard rule - about 2/3 down from your recommended highway pressure. So, if around 30 psi is your highway pressure, the psi setting for sand should be 10 psi.
If you drive in soft sand a lot, taller wider tires are a good investment. They have a much higher volume of air and can be aired down more easily to very low psi.
Tires with an aspect ratio of 50 or lower (like a 255/50 R 16) have very low volume of air and should neither be aired down much, nor be used at the beach in soft sand.
The image below shows the footprint of two equally heavy Jeep Grand Cherokee. Both riding on tires of the same size. One with BFG All Terrain (AT) tires at 32 psi - the other with BFG Mud Terrain (MT) tires at 10 psi. It is very obvious that the MT tire barely breaks the surface, whereas the AT tire created a deep channel.
By the way, don't listen to guys telling you that MT tires are bad for sand. Its not the tire design but the low tire pressure that is important - and your gentle foot on the gas, of course.
The next image shows also that the aired down MT tires have more directional stability. They drive dead straight. The full pressure AT tires wander at their own will. Even when the driver holds the steering wheel straight, the tires will follow their own crooked path.
Here is another very valuable tip for driving in soft sand: Drive as straight as possible! Driving straight makes the rear tires follow in the same already compacted channel the front tires have created. By initiating a turn, the rear tires leave the channel and each of them will now create their own channel and thus will have to fight a small berm of sand each. That increases the resistance by 100%. Increasing the chance to get stuck. The tighter the turn the higher the resistance.  Here is more...
Back to driving on soft sand.
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4x4 driving tips