You wash your head (hopefully) which is exactly why you should wash the place you rest that head at night. Hair, sweat, drool, it all collects into those pillows of yours.
Better kill ALL the germs so you won't get harmed any time!!!!!!!111
(Might affect your immune system and cause allergies, but what the heck, hygene product manufacturers have to earn their money, don't they?)
Regards
Your hygene industry