What is the origin of the idiom "icing on the cake" and when to use it?

The phrase "icing on the cake" has its origins in the practice of adding a layer of icing or frosting to a cake to make it more appealing or enjoyable. The phrase first appeared in print in the 19th century, and it quickly became a popular idiom for describing something that enhances the enjoyment or value of something that is already good.