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.
Chances are if you're over 30 that backside of yours is starting to look like two gluten-free pancakes, even if you train. Here's why and how to fix it.