According to Susannah Gora's book You Couldn't Ignore Me If You Tried: The Brat Pack, John Hughes, and Their Impact on a Generation (Three Rivers Press, 2011), actor Matthew Broderick was apprehensive about all of the fourth wall breaking he'd be doing as Ferris Bueller. "To be perfectly honest, when I read the script, I didn't get it," Broderick confessed. "I said, 'It's all these monologues to the camera—what is this?'"
What it is, Shmooper, is a means for us to understand Cameron. The majority of Ferris's monologues are about his tormented best friend, and they supply insights into Cameron's character that we wouldn't be able to get otherwise.
The most outstanding example of this is Ferris's men's room monologue at Chez Quis, when he describes Cameron's "twisted" home life to the audience. "That's why [Cameron's] sick all the time," Ferris explains. "It really bothers him. He's the only guy I know who feels better when he's sick. If I had to live in that house, I'd probably pray for a disease, too." This is premium grade perception into what makes Cameron tick, and there's no way we're going to get the goods without Ferris delivering them to us directly.
It's also worth noting that Ferris is honest with the audience. When he speaks to us, he speaks the truth. The same can't be said for the other characters in the movie with whom Ferris—the first-rate fibber, falsifier, and fabricator—interacts. He may lie, cheat, and steal his way through his day off, but he always seems to keep it real with us.