Presumably, if one is going to be keeping an aircraft on the ground for a long time without running the engines or the APU, the aircraft's fuel tanks will need to be drained and purged...
- ...to prevent potential damage upon return to service due to degradation of the fuel over time...1
- ...to forestall a potential explosion hazard from fuel vapor in the fuel system and coming out of the tank vents on hot days...
- ...and to protect the local environment, both from the aforementioned venting of fuel vapors and from liquid fuel potentially draining out onto the ground should a leak develop while in storage.
However, on the flip side, as @Bianfable's comment points out, defuelling an aircraft has a significant disadvantage of its own; empty fuel tanks are vulnerable to condensation, which can cause accelerated corrosion and (in colder climates) ice accumulation.
Piston-engined avgas-burners generally have to be defuelled if they're going to be sitting inactive for six months or more; how long, generally, can a turbine-engined aircraft be left on the ground with jet fuel in its tanks?
1: Removing the fuel at the end of the storage period would also work for this purpose, but that would a) waste a planeload of fuel and b) involve draining the fuel system of various tarry fuel-degradation byproducts rather than just ordinary fuel.