You decoct a decent portion of the mash. So on your first decoction, you want to convert the disolved starches prior to boiling. As Gnome states, it probably makes it easier to ensure full conversion of the entire mash.
Additionally (I don't know if there is science to back this up), it seems to me some of the potential flavor enhancements of decoction mashes might be due to the long cooking time of the sugars. Conceptually, boiling starches doesn't seem like it would taste as good as boiling sugars. But, there are those that dispute the taste benefits of decoction. Apply as many grains of salt as you think are appropriate.