Many ethnic instruments use beads like this to help "fine tune" the strings. They do not have machine tuners like what we have on more modern instruments. A lot of times, it's just a friction peg (like on a violin). Even some violins have little tuners on the bridge for these subtle adjustments. That's the purpose of the beads. A slight move of the bead stretches the string a bit raising/lowering the pitch.

Tom Tracy
http://www.soundclick.com/tomtracy