The main job of punctuation is to eliminate the ambiguities and garden paths that would mislead a reader if print consisted only of vowels, consonants, and spaces. Punctuation restores some of the prosody (melody, pausing, and stress) that is missing from print, and it provides hints about the invisible syntactic tree that determines a sentence’s meaning. As the T-shirt observes, punctuation matters: Let’s eat, Grandma has a different meaning from Let’s eat Grandma.
The problem for the writer is that punctuation indicates prosody in some places, syntax in others, and neither of them consistently anywhere. After centuries of chaos, the rules of punctuation began to settle down only a bit more than a century ago, and even today the rules differ on the two sides of the Atlantic and from one publication to another. The rules, moreover, are subject to changes in fashion, including an ongoing trend to reduce all punctuation to the bare minimum. They fill scores of pages in reference manuals, and no one but a professional copy editor knows them all. Even the sticklers can’t agree on how to stickle.