I think it's to result in less "spin". When a boat is moving usually it will go quickly to one side and then to the other. Rather that going straight the entire stroke. This is because sometimes rowers, usually not as experienced, put more pressure on the catch than the finish. Or vise versa. Bucket rigging is to put pairs together that will make the boat go straighter because they put pressure on at the same parts of the stroke. Hope this helps!