The lat pulldown and pull-up have a lot of things in common, but they are not the same exercise. They both train the lats through shoulder adduction. However, with the lat pull-down, you’re pulling the bar to your body. With the pull-up, you're pulling your body to the bar.  Although you can use both exercises to build and strengthen your lats, the pull-up is a more functional movement. Plus, more muscles are involved when performing a pull-up compared to a lat pulldown. If you need or want to get better at pull-ups, you should be performing pull-ups, not lat pulldowns. On the other hand, if you just want to target your lats, lat pulldowns are just fine.