I know that the winglets on planes like the Boeing B737 and Embraer E-190 reduce fuel consumption, but do they do more than that, or are they just for the stated purpose?
The short answer is they exploit the flow circulating around the tip by generating lift that is oriented toward the fuselage, with a slight forward component because the local flow is slightly inboard as it curls around the tip, and lift is 90 degress to the flow. They are basically sails, and were originally called "tip sails". The result is a small thrust component, the same thrust component that propels a close hauled sailboat forward, and there is a downwash from the vertical wing that, being vertical, is actually an "outwash". This outwash effectively turns some of the circulation field back in the other direction to a certain degree, weakening it.
They take the energy in the circulation and do useful work with it, in other words.