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.
In four months I'll be a whopping 28 years old. I know a thing or two about training and all that goes into it, but I also respect the fact that there's a hell of a lot I don't know enough about, and other topics I need to know something about that I'm not even aware of!