Some lifters have trouble hitting their backs, even with back exercises. Often, the biceps take over. The unilateral lat pulldown allows you to really focus on the back one side at a time.
Do not fully extend the arm at the top of the movement.
Hey, wanna tick a lot of people off? Want to start a flame war, a heated academic discussion, or just a good old-fashioned penis-waving contest? Then come out publicly and say that total body training is better than body part split training.