Nobody likes blowing sand to get in their engines, it wears things.
regarding windmilling fans in the wind. It all depends on whether they have those loose shrouds/clappers/snubbers or those funny thingies that are partway out the blades and touch each other.
If they are present (most new designed engines no longer have them) then the fact of their looseness (at ground windmill conditions) makes them clatter when they fall over at the top and bang into their mates. If they are very loose then the clatter is quite pronounced and can actually wear their surfaces the bang. hence the need to keep them out of the wind.
Even the new engines with none of these thingies partway up the airfoil may rattle at the joint with the hub that they are mounted in. Too much rattle = too much wear.