I can use a "pointed finger" to show you something. If I just say "this is my car" it may not be obvious which car I am talking about. But if I add a gesture with my hand and point at the car with my finger it becomes very clear what I'm talking about. Because a "pointed finger" is a good way to direct a person's attention towards something.
But what does it mean when the author says that a pier stretched into the ocean like a pointed finger?
If I wrote "the pier went into the ocean", it doesn't produce any sort of feeling. All that sentence does is state the facts. Your author is trying to stir up some emotions inside of you. The pier "stretched" into the ocean. Like when you stretch your arms above your head and the muscles lengthen and it feels so nice. And when you point at something, like a car, you immediately focus your attention on a small specific thing. But this finger (the pier) is pointing out at the vast ocean! It's like pointing into the sky! So there is a direct contrast between what you expect (a small specific thing that you would normally point at) and what you get (the vast ocean). This contrast makes the sentence more enjoyable and helps to invoke the feeling of the ocean.
That's how I think about it anyway.