What can happen is, you become too close to your material to actually see what is actually going on. Being close is a good thing, but it is just possible, because of the closeness to the material, to miss the blindingly obvious. Someone who doesn't have that closeness may spot things that you may have inadvertently overlooked.

Just having someone else to have a look may expose deficiencies. They may also be able to make suggestions you havn't considered that can result in a better song overall. It is one thing to "decide" to have something a certain way, it is entirely different thing to have something "just occur" a certain way. The difference may be the difference between a "Good" song and a "Great" song, but for the want of a fresh perspective.