Winglets are there primarily for induced drag reduction.
However,
By increasing the efficiency of the wing outer section they also increase lift by making the wing "virtually" longer and also therefore increasing the aspect ratio.
Also, I read an article when I was an apprentice about wing "sails", these were described as long thin winglets which reduced drag on smaller slower a/c and were more efficent than winglets over short distances. Somewhat like the B737-800 BBJ.