Airports/Airfields = Lots of noise and stress going on, easy to get hurt if you're not cautious...

Its to distract birds and warn personnel that the engine is on (the prop tips are there to warn you the outter edge of the props)....to the people saying "they would hear it". The simple answer is: they are wearing very good ear protection and sometimes there are more then one engines on or planes moving around in the area.

(Bird: ) Hey, a big metal thing is coming my way, let's get out of here! Oh wait... look at that spirally thing in the middle... can't.....get.....my...... eyes off of it..... too distracted..... to evade *splash*

Morale: I think it didn't have a lot to do with birds