Even if The Souls didn't have spookily unnatural knowledge about famous fathers or the Native American tribes of New York State, they'd still have even better knowledge: knowledge of the self. All these journeys that they've been on throughout The View from Saturday are basically just the old "finding yourself" story. And what The Souls figure out is that, if you know who you are, you've already won.
In The View from Saturday, Nadia, Noah, Ethan, and Julian already know who they are before becoming The Souls.
According to Konigsburg, becoming The Souls allows Nadia, Noah, Ethan, and Julian to become their best selves.