I've got a dumb question, but I've seen a lot of this lately:
People riding around with drinking straws sticking out of their bike helmet...why?
A few suggestions:
1. So that you know if anything overhead is too low
2. Sophisticated cooling system
3. Do-it-yourself radio antenna or to improve mobile reception
Drinking straws, pipe cleaners, long zip ties - they are all used to ward off black and white birds which have been possessed by evil spirits - and they all look completely naff (trust me, I am an expert on looking completely naff).
If you want to decorate your helmet, aspire to something like these: