"Your work ethic is like icing on the cake, my dear!"
"That last piece of chocolate was really icing on the cake."
"Enjoying a cold margarita after a long day is icing on the cake."
"Meeting new people is like icing on the cake, especially if they help you out professionally!"
First off: What is icing? It is a thick confection made of sugar, fat (like butter), flavor additives (like vanilla extract, fruit, liquor, etc), and eggs to top and decorate cake. Can you eat cake without any frosting? YES. Is it better with some sort of frosting? YES. It adds moisture, more flavor, and a smooth texture in conjuction with the delicious cake. Cake and frosting go together like freshly squeezed moijiots and Seattle's first sunny summer day.
Since I was a little girl, surrounding myself with my E-Z bake oven, cookies, cake, and brownies....the expression "icing on the cake" was tossed and turned around within the everyday conversation between people. Bosses used it to compliment their employees; People said it when a positive gesture passed through their day; Teachers said it to well-behaved students.
After seeing how people loosely use the expression, "icing on the cake," it is meant to compliment a genuine, absolute, and clear act that positively adds to your life. Is it necessary? No. Is it needed on a hourly basis? No, and you won't come across it every day of your life. Does it make you smile and look at life from a different view? Yes. Small gestures from total strangers will bring the best in you. It will make you want to do "icing on the cake" acts to every single person you meet.
Moral of the story? Just like icing, petite and humble acts, give us more to our day-to-days. We may not need an extra chocolate chip cookie, a scoop of pistachio gelato, or a glass of a strong Malbec, but unneccessary modest moves are pure sweetness.
Now: go forth and put "icing on the cake" to someone's day.