As an experienced wedding photographer, I can tell you that most couples need a lead time that can be eliminated with an engagement session. So on your wedding day no stranger will knock on your door armed with a bunch of cameras, but you will greet your wedding photographer already as a friend, which will have a positive effect on the pictures.
You can use the resulting photos for the invitation cards, the Save the Date cards or your wedding website, for example. The pictures can also be mounted on canvas or framed in your living room and will certainly become an eye-catcher.