Duncan is averaging 10.9 points per game, and San Antonio's defense is statistically better with him on the bench. But there are so many unquantifiable benefits to having him onboard, and eventually they'll translate to more wins.
So what exactly is Duncan's role? We don't know, which is a credit to Duncan. He can score, though he hasn't needed to this season and likely won't shoulder much of the offensive load. He's fifth on the team in usage rate. He can rebound, pass, knock down the mid-range shot, create a fast break opportunity, operate a pick-and-pop, set a crushing pick to free his teammate, protect the rim, hug Tony Parker, whatever. Even at his advanced age, there isn't anything on a basketball court that Tim Duncan can't and will not do. There are not many better defenders than 39-year-old Duncan. That's not hyperbole, it's basketball fact.