I thought I remembered reading something about this, so I dug up one of my books. In the volume entitled "Modern Combat Aircraft Design", Klaus Huenecke says the following, and I quote:
The saw-tooth leading edge is a sure sign that the aircraft builder has not got the wing flow under control and therefore has to resort to this makeshift solution.
A fence is a very easy means of influencing the boundary layer. At one time this was frequently employed, but because of advances in wing design it has largely disappeared from combat aircraft.
By means of a boundary layer fence a virtual division of a sweptback wing into discrete areas occurs, so that undesirable spanwise boundary layer effects cannot spread, except to a limited extent, to neighboring areas. In particular the object is to prevent the drifting of the boundary layer into the outer areas which are endangered by separation anyway. A disadvantage of the boundary layer fence is its high friction and interference drag in high speed flight.
I'm not sure how much of this applies to commercial transport aircraft, but surely some of it has to be relevant. Hope this helps.