I think of them as ways to insert personality, too, Louise, either the characters from the way they phrase something or do something or for your own personal writing style, too.
There are lots of things you can have pre-prepared other than just who the characters are, what their backgrounds are, how they dress, how they look, how they act, how they think, etc. or what happens next in the storyline, though all of those things still come into consideration. And, granted, they are the first things that everyone thinks about. This other stuff — accessories!