A scuba diver needs pressure in an air tank because water pressure increases and pushes down on their lungs as they go down. The air in your lungs is more compressed and more is needed to fill them...
The average human uses about 25 litres of air per minute (at sea level) and since a scuba tank is between 10 and 15 litres, if the gas wasn't compressed, it wouldn't even last a minute. By...
The air inside SCUBA tanks is, usually, just regular air that has been compressed and "jammed" into a tank. Some diving applications utilize gas mixtures of oxygen, nitrogen (and sometimes hydrogen).
Most recreational SCUBA divers dive using an on demand regulator system. As the diver inhales, the low pressure created in the second stage regulator (the thing nearest the mouth) acts on a diaphragm...