Sure, it may sit in the depths of HR until the end of time, but you can be sure of two things: One, your former boss (and future reference) will read it and be impressed. And two, if you ever decide to come back to your company (hey, it happens), it's a very good thing that last thing on file is a great, professionally written resignation letter.
Your End Game. When you hand in your resignation, it can kick off a bit of chaos__your position will need to be filled, but your duties will also need to be covered until they can replace you. As part of your resignation letter, it's a good idea to convey what your availability is during that transitional period.