Much like HBO, the esteemed producer always takes care of his own. When walking by any poster for an Apatow-directed, produced, or written flick, it's likely you'll see one of a handful of familiar faces glancing back. Once an actor's been initiated into the Apatow "family," they're effectively set for life. Just look at Jonah Hill or Paul Rudd. Hill was far from the conventional leading man and Rudd had a track record of appearing in chick flicks.
Thanks to Apatow, who's become more than a name, but a brand, once-obscure bit actors have become stars and those who've been out of the limelight for a minute come to be hailed by millions across the country as the funniest actors in Hollywood. The chance to flex those comic muscles in Apatow's movies is invaluable (especially since it means you'll probably be guaranteed work in every last one of his movies until parted by death).
Cusack, a funny guy in his own right, could profit big-time from a meeting with the hit-maker. Even a quick cameo (which, in the dude's movies, can prove to be some of the most memorable moments) could be the jumping-off point for a long and prosperous relationship.