To keep groundwater out. it's common to use nitrogen, but the LCC would be hard to resupply with tanks[0], so (based on the photos) they used air that had been dried.
Story time: The airbase I was stationed at in Germany had been built by the French, prior to their partial departure from NATO[1]. There was a buried cable on base that went bad while I was there. It was pressurized but apparently the hole in the casing/jacket was large and admitted more water than could be kept out by the nitrogen. So the airmen of the outside plant had to use their TDR[2] to find the fault and dig it up to repair it. To their surprise it turned out to have paper insulation[3], as it was made before plastic began to be widely used in wiring. It also had swastikas printed on it - because the French had used captured Nazi war-surplus cable when they built the base in the late 1940's.
[0] It would be hard to maneuver large & heavy steel cylinders down the entrance, through the blast doors, and satisfy the security concerns of the LCC crew that they were legit. And then remove the empties afterwards back through all that. The cylinders can not be stored on the surface, for security reasons and because the buildings would no longer be present after an attack.
[3] The water soaked much of the paper for quite a distance in either direction of the hole. So they ended up replacing a rather long piece of cable instead of just putting in a splice junction. Lots of digging & cursing needed.
We didn't have that, but they do. If you look at one of the photos the panel in the rack has a pressure alarm for the cables. So the crew would be alerted should the pressure change.
> This is the basic premise of cable pressurization: Keep the pressure within the cable in excess of the pressure that could be applied by standing water.
For people like me that didn’t understand why the cables are pressurized.