When you get cake to eat, it is a pretty good deal. If the cake has frosting or icing on it, then the deal is that much sweeter. When you say that something is icing or frosting on the cake, what you are implying is that the addition or inclusion of something makes a really good situation even better. A great deal, becomes a wonderful deal. When used approvingly, “icing on the cake” is used to refer to an unexpected extra good thing to have happened when you have already had some luck.
I was thrilled to get a big bonus. When my boss told me that I had also been promoted, it was really icing on the cake.
*Geetha was pretty excited when her book was published — winning the Pulitzer prize was icing on the cake.
The expression can also be used to show disapproval. There is no point in adding anything to something that is already good. The thing that you have added is unnecessary — icing on the cake.
*Having a glossy cover for our new magazine is both unnecessary and expensive — icing on the cake.