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.
A lot of you have been training for years, and as such have experimented with dozens of training methods. Most are probably long since forgotten, like the girl with the braces and padded bra that you dated once or twice back in high school. There wasn't any chemistry and you went your separate ways.
Understanding the science of sets, reps, and muscle-specific fiber composition will make you sound smarter. Going to the gym and applying that knowledge will make you bigger.