When Spotify Technology SA went public in April, Goldman Sachs Group Inc. was sitting on a stake in the music-streaming startup worth more than $350 million, a sevenfold return on a 2012 investment.
Those gains weren’t the handiwork of any of the hundreds of Goldman professionals whose day jobs are to invest in companies. They came instead from the firm’s bankers, who have been quietly moonlighting as venture capitalists.
These...