From where the lake is situated, there is still plenty of fairway past it, the fairway wraps around the lake to the green.
I don't think it comes into play at all on the approach for the better player--but it does on the tee shot, as evidenced by Fred Funk earlier today. I'm not sure what the distance is from the tees to the edge of the hazard.
I doubt this was a feature in the original Oakland Hills when Ross laid it out--did RTJ dam a small stream to create it?
I would just classify it as a simple risk reward--i.e. a drive down the left, into the 'neck' of the fairway, gives the player a shorter approach--but a player laying back short of the water will be left with a slightly longer approach.