As mentioned before wing fences do prevent side slip.
However winglets do as follows. The vortex derived at the wing tip by high pressure air escaping from below the wing tip to lower pressure above the wing is much reduced consequently reducing drag. However the vortex that is generated at the tip of the winglet now lies above the wing. This vortex being of greater velocity than the air around it produces additional lift. Winglets also prevent aileron reversal again buy preventing high pressure air escaping to the upper surface of the wing reducing aileron reversal at high speeds and the necessity for inboard mounted high speed ailerons.