To cool and dehumidify the interiors
Except they don’t need to empty the tank every 6 hours.
As you probably know, when an aircraft gates, it shuts off all engines including the APU. That means it relies on external power for electricity and HVAC.
The problem of hot interiors
Ever get in your car after it’s sitting in the hot sun, and it’s freakishly hot and you immediately start the A/C? Try not doing that, close the door, leave the windows up and sit there for 5 minutes. You can’t. It’s too darn hot. That’s why it’s so lethal when a baby is left in a car. I’ve had PVC pipe, left in the hot car, actually bend into a pretzel.
That’s caused by the sun bearing down on the car. It’s called “solar gain”. The same thing happens to railroad cars; that’s why railroad museums only have 2 kinds of cars: open-window, and air-conditioned.
For the auto industry to design interiors that can endure that for 20 years and not melt and bend like my PVC pipe, *is actually a really hard problem, and they spend billions on it. Obviously it’s the top priority in interior design.
Now turn the page to an airliner. Exactly the same thing will happen due to solar load - except aviation interiors have a different top priority (safety and weight). So they must compromise away automotive traits to get what they need.
Mind you, the best places to store airliners are dry and very hot - like the Mojave Desert or Tucson. This makes the solar-load problem even worse.
So we have a) a vulnerable interior, b) very hot storage locations, and c) you can’t open the windows on an airplane.
That means you must run HVAC, or remove the interior, or lose it!
Damp interiors are just as much of a nightmare, as they will breed mold. An unmaintained interior can accumulate moisture due to condensation: hot humid air leaks in during the day, then the humidity condenses in the cold night. Now you have liquid water where you did not expect.
Cooling (and dehumidifying) an airplane
Aircraft are already set up to take external sources for a few things: direct HVAC, compressed air and electricity. That’s so they can power down entirely at the gate for smog and fuel savings, and rely on shore-side equipment to keep the aircraft comfortable.
If they can hook up HVAC hoses (conditioned air) directly, that’s grand; however the airplane can make its own HVAC out of compressed air, thusly
The HVAC packs are designed to work on “bleed air”, compressed air drawn off the engines. Shore-side compressed air works just as well. The same thing happens whether shore-side or from engines: compressed air is very hot, and water-laden. It has the same air-mass:water-mass ratio as it did before it was compressed, so much more water per CC, and the hot air can hold that.
But when you cool compressed air, you reduce its capacity to hold moisture. The moisture condenses at this point, and every compressor and intercooler has gear to contend with this (cheap home compressors have a drain on the bottom of the tank).
Now that the compressed air is cool, if you allow it to decompress, it becomes cooler still. (This is just the reverse of the air heating up when compressed; it’s the poor man’s version of the freon cycle, same basic physics but not benefiting from the vapor-liquid enthalpy).
When you use an air-powered hand tool vigorously, it gets quite cold.
Since we’ve already wrung most of the water out of this air, when we decompress it, it comes out exceedingly dry compared to the air we started with at that same pressure.
Thus, we both air-condition and de-humidify the airplane in one stroke.