At first glance, punctuation might appear to be an aspect of written language only. But much of punctuation is essentially a guide to reading aloud: a punctuation mark can represent a pause, or a tone of voice.
Punctuation can also play a part in the meaning of a sentence. In the title of the book Eats, Shoots and Leaves, the presence of the comma completely changes the meaning of the phrase, and in reading it aloud, the comma marks a pause. (You may also note that the change in meaning depends on two homographs of "shoots".)
Finally, punctuation is often a matter of style. Choice of punctuation, as much as choice of vocabulary, is a personal taste.
There is a leaflet in the SAS series which amalgamates all the advice on punctuation in one page: see Punctuation (SAS leaflet).