Baseball’s San Francisco Giants are selling a 10% ownership stake to a private equity firm — joining a growing trend in American sports. The firm in question is San Francisco-based Sixth Street. All major sports leagues in the U.S. now allow those firms to invest in their teams, according to the New York Times.

Historically, major sports teams in the U.S. have each been owned by one really rich person or family and the only way for them to cash in on their investment has been to sell the whole team. 

But as the value of sports teams has risen, owners have realized that selling off part of their stake in a team “is a way for existing owners to capitalize on the rising valuations of their clubs,” said Tim Koba, an assistant professor in sport, event and hospitality management at High Point University.

Right now, private equity firms are willing buyers.

“Essentially, they’re buying into people’s passions,” said Stefan Szymanski, a professor of sports management at the University of Michigan. 

Because fans are so passionate about their teams, they’re willing to sink a lot of money into watching and supporting them.

And private equity, Szymanski noted, sees a revenue opportunity. “What they’re trying to do is trying ways to extract more of the financial value of that happiness from the fans.”

By raising ticket prices, he said, or finding new ways to reach — and charge — fans through digital channels.