Why do you add air to the ballast tanks in a submerged submarine?

Air is added to the ballast tanks of a submerged submarine to increase buoyancy and facilitate surfacing. By replacing water in the ballast tanks with air, the overall density of the submarine decreases, allowing it to rise to the surface. This process is essential for controlled ascents and maintaining stability while submerged. Additionally, managing buoyancy is crucial for operational maneuverability and safety.