In the film, Joel (Cruise) is super stressed about getting into his college of choice, but also overwhelmed by the newfound freedom of being left home with no parental supervision. Dilemmas ranging from "Should he drive his dad's Porsche?" to "Should he turn his home into a temporary whorehouse?" arise, and Joel deals with every single one in the exact same way: By doing adorable things while wearing Ray-Ban Wayfarer sunglasses.
He most famously puts them on right before uttering the film's catchphrase (and the '80s version of YOLO), "Sometimes you gotta say what the f**k." That he dons those particular shades at that moment is no happy accident.
In the 1980s, sales of Ray-Ban's Wayfarer sunglasses had sunk to an abysmal 18,000 a year. In an effort to turn things around, the company offered the producers of Risky Business $50,000 to feature the Wayfarer model in the teen comedy and, just as importantly, on the movie poster.
Sales of Wayfarers immediately took off. By 1989, Ray-Ban had sold more than 4 million pairs of the now-iconic model, a pretty sweet return on their investment. But the Cruise promotional juggernaut had just gotten started.