if you understood how wake turbulance is produced and how this causes excess drag, the winglets are there to prevent some of the wake turbulance and gain some extra fuel economy. Not noticable on short haul, but long haul definatly.
The basic principle is because of lift, which is high pressure on the underside of the wing and low pressure on the top of the wing, the air naturally wants to flow to the top of the wing where the low pressure is. As it tries to do this it overlaps the outer edge of the wing causing it to spiral as the wing continues on the flight path.
To maintain level flight and level speed, thrust must equal drag, so more thrust (fuel) is required to compensate for more drag. With the winglets the theory is you will need less thrust to maintain the same speed because you have less drag. Although the cost of retrofitting and certification sometimes is more than the savings that will be recovered, which is why it is more viable on the long haul fleets
I believe the Winglets still are a option on most A/C aswell, so the carriers decide if its viable for the winglets for the route they are purchasing the A/C to fly.
Look at most of JAL 743's and 4's, alot of them are shorthaul only
Hope this helps
4s