the words help bring the piece of writing to life. they may help in creating imagery, and they may even help the reader feel like they are involved in the story. the choice of words can also help set the mood for the story like mysterious, happy, sad, etc.
You cannot put any sort of endmark (period, question mark, or exclamation point) here because it is not a complete sentence. Complete sentences must have a subject and a predicate. You can tell there's not even a verb here, so this is just a fragment, and we would have to use a comma.
a. Very truly yours,
