Well having all of the map discovered is an advantage all in itself, but other than the obvious there is actually no benefit to having the entire map revealed. When you reach the final tower and uncover the map, you will get a message to say that you have done so and that's it.
I will admit I was a little disappointed. I did expect there to be some sort of perk added to your Shikah Slate as a reward of some kind, but you won't get anything for achieving this. Don't let it be an issue though, there are still more than enough secrets left on the map to keep you going. At least now the entire map has been revealed so you now know where to go right?