For the tax breaks.
It's simple: As an airline depreciates an aircraft, they get a tax break because it shows up as a loss on their profit/loss statement, and is reflected in their balance sheet.
An airline with 100 fairly new aircraft in Mohave can actually cut its taxes in half by a) leaving them out there to rot and b) buying new planes to cover the route, but also showing depreciation.
Many of NW
's DC-9s were depreciated years ago. That's why they're still flying. They can drop off new
aircraft in the desert and depreciate them more quickly while using the DC-9s that are, technically, worthless. This also saves on insurance, since the replacement cost of a DC-9 is next to nothing (maybe $1M for a parked, opeational DC-9).
Like many things in accounting, it's a farce accepted as a Generally Accepted Accounting Principle. They're allowed to do it simply because there is no way to actually measure depreciation, but it does exist and is something that needs to be dealt with. So each company chooses an accepted method and depreciates the planes. If they're sitting out in the desert, they don't use fuel, and they do give you a tax break... Nice, huh?
Those who fail to learn history are doomed to repeat it in summer school.