I'm sure Jay is looking at this as a long term thing. He's is in it to make money sure, but the bigger picture is the name/brand of RocNation. Right now they probably have 2-3 athletes, but if/when they expand to 25-30 athletes, then the overall perception grows big time. Then the money starts really rolling in. It won't matter than CAA is handling the work, the Roc Nation brand will be the one standing out. Don't believe me? Well, when you hear these signing being reported on ESPN/elsewhere, wht do they say? They say, "(athete name) has signed to rapper Jay-Z's sports agency" Nothing about CAA unless you dig deeper on ur own.
On top of all that, the athletes see a "personable or caring" individual in Jay-Z that they identify with moreso than a stoic old white dude they were shuffled into signing with. That said, there's ALOT of good white agents out there who are honest and care about the client. But the overall image is that they are shady.
Jay is pretty much just a CEO/figure head. He's not doing any negotiating. He probably sits in on the meetings with the athlete and cultivates a relationship with them over dinner, etc.
But the bigger picture is gonna be a win for him/Roc Nation, so he's not a puppet in that sense