You need a groove on a board. Easy, peasy, you look at the grain and choose the best direction to plow. Clamp the piece to the bench and plow your groove. But what if you need another groove on the other side of the board. How do you do this? If you just flip the board around, you will be going against the grain, and make an ugly cut.
Am I missing something? How do you get a groove on both edges of the same face of a board if you can't afford a right hand and left hand plow plane?