He could play the three in small-ball lineups, where he could hide on the opposing team's worse player. Not only would be able to space the floor better than Leonard (allowing Kawhi to play inside the arc more), but he'd also be able to break out more of his game against guards.
One thing that was really good about him is that he moved a lot along the perimeter. The Spurs don't really have anyone who does that besides Green.