Ferris Bueller doesn't narrate. He breaks the 4th wall. That's a whole 'nuther thing with a tradition going back to Seneca. Characters breaking the 4th wall (a term that goes back to Moliere) occupy an entirely different purpose than narration, which started with the Chorus. It's also a whole 'nuther can of worms in that the minute you start breaking the 4th wall you're delving into metafiction, for better or worse. Voiceover, on the other hand, is a narrative shortcut.
He's providing asides and insights. It's much in line with what John Hughes did in Breakfast Club - he had Anthony Michael Hall deliver a greek chorus like intro and outtro to cast the players as larger representatives of society. It's very much a "and these children that you spit on as they try to change the world" approach. None of what follows is relevant to the plot. It's all an aside which exists solely to increase our affinity for Ferris.