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.
Moderate weight or moderate sets? Use more reps per set, or use heavier weight, more sets, and fewer reps per set? God almighty, which is the optimal way to train for size?