Like a shock absorber, it dampens the movement of the steering head. Sportbikes have them because they have aggressive steering and it prevents the "death wobble".
I seriously doubt any HD needs one and the R3 doesn't need one either.
They are used to smooth response and limit high end. I had a 2005 and the bike ran better with better low end response and there was no high end limiting. I was happy with them removed, and it was pretty easy to do.