Clearly, this cycle has demonstrated that a governor's endorsement is vastly more helpful than the endorsement of a U.S. Senator. This makes sense; Governor's have organizations and machines (think Ed Rendell). And they actually live in the state.
... So this isn't merely a PR move, it could actually have electoral ramifications.
Remember, Hillary doesn't need to win NC. If she wins Indiana, and comes close in NC, it's a "win" ...