An LOE serves a couple of purposes. You fileis good enough in the eyes of WP that they want to encourage you to finish your file ( the sooner the better). Also, it enables them to offer you an overnight visit (which you should do).
To receive an LOA is not impossible, but improbable, unless something in your file changes that is really impressive, like a 36 ACT.
LOA's are not as plentiful as they have been in the past. They are mainly used for recruited athletes or other highly desirable candidates. That is why the LOE was invented. It's meant to encourage the candidate to complete their file and to offer an overnight visit.
It means that today you are currently in the game and sitting in a good place. As the letter says, they want you to complete your application.
If you notice I said today. You need to keep working hard to complete and improve your file. It just takes one person to beat you out.