Why do people dip their finger into the leftover havdalah wine?

It is customary is to extinguish the havdalah candle by dipping it in the leftover wine that accumulates in the cup's saucer. This demonstrates that the candle was lit for the sole purpose of performing the Havdalah ceremony and since the ceremony is now over the candle is extinguished.

Many have the custom to dip their fingers into the spilled wine and rub them above their eyelids. This demonstrates how mitzvot are so precious to us. King David writes1 "The command of the Lord is clear, enlightening the eyes."2

There are also those who have the custom of placing their wine-dipped fingers in their pockets, as a segulah (spiritually propitious act) to become wealthy.