Why do some bikers wear a kerchief on their face?

It's to make them look badass..

I have a friend that wears one when the bugs are bad or if it's raining. He says it takes some of the sting off. I tried it once in a rain on his suggestion, I didn't like it. Kept falling off, just a general pain in the butt. I guess to each his own. When it's cold out I do wear a face warmer though, but not quite what you are talking about.

Try riding with an open face helmet or brain bucket and tell me if you like the taste of bugs and you will have your answer. Or perhaps you want to wash of dirt and bugs off your face every ride.

Depends on the individual biker, how exposed they are to the sun and wind, how sensitive and fair their skin is, and how long the ride with exposure to sun and wind to prevent a nasty wind burn.

Myself:

Forty years ago on a seemingly warm but low humidity December morning going to college campus I frostbit the moist parts of my eyelids...It's been goggles and full face helmets ever since.

Obviously, you've never driven a bike before.

When you ride a bike down the road, at say 40 MPH, AND you don't have a windscreen (mounted on the handlebars) to deflect "objects" and "debris" that's in the air - It has a tendency to bounce off of your face. So, you have to put something on the cover your face from the stuff.

the reason for this or one of them is to keep bugs out of your mouth and nose another is some times for the cold hope this helps

To keep debree out when they are speeding down the freeway...