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.
Sometimes it's really interesting to dig into the microscopic details of training and nutrition, to dissect the body of academia and examine every little study. That's how we learn things. That's how we refine the science that later helps us get bigger and stronger and leaner.