Purpose of a Metonymy
As with other literary devices, one of the main purposes of using a metonymy is to add flavor to the writing. Instead of just repeatedly saying, "the staff at the restaurant" or naming all of the elements of a dinner each time you want to refer to the meal, one word breaks up some of that awkwardness.
Using a metonymy serves a double purpose - it breaks up any awkwardness of repeating the same phrase over and over and it changes the wording to make the sentence more interesting.