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.
For years, I've been watching zombies, training zombies. They wander aimlessly from station to station, doing exercises haphazardly with really no rhyme or reason except that, "this one kinda' feels like it's doing something." Once in awhile, they'll grasp on to a particular program but they'll end up beating it into the ground, doing it for months and months until they've long since milked any effectiveness from it.