Why should scuba tanks always have some pressure in them?

Having some pressure in scuba tanks is essential primarily to prevent moisture from entering. When a tank is filled with gas, it creates a barrier that keeps the humidity and moisture in the ambient air from coming into contact with the interior of the tank. This is particularly important because moisture can lead to corrosion and contamination of the gas, which can affect the functionality and safety of the diving equipment.

In scuba tanks, the presence of pressure not only serves to maintain the integrity of the tank but also supports the overall safety and reliability of the breathing gas, ensuring that divers can rely on their equipment during dives. The other aspects related to scuba tanks, such as buoyancy changes or regulator functionality, are influenced by different factors associated with diving physics and equipment design but do not directly connect to the moisture control aspect that comes from having pressurized gas within the tank.