Glory
There's the glory of seeing someone you manage rise to the top, which is nice. But mostly you'll just be consumed with the glory of stuffing those wads of cash into your genuine cashmere pockets.
Many managers also become producers somewhere along the line, which can be a glorious transition for you if you'd prefer to be more involved in the movie-making process, and not just in the movie-casting process. A star will often lean on a studio to pay a manager a producer fee, and voilà—you're a producer. It's like magic, but without all the doves.