Before the advent of machines, wide-grip pullups were the exercise of choice to build broad, flaring lats. But they are hard to do for most. With lat pulldowns, resistance can be accommodated to allow for proper execution.
Lat pulldowns put you in a fixed position, allowing more focus on the lats and control over the amount of resistance you want to use.
Pullups have been considered a traditional width\ builder, especially when executed with added weight.
A study at James Cook University compared lat pulldowns with pullups using the same grip and hand position. Both exercises effectively activated the lats. Pullups produced greater activation of the biceps and spinal erectors.
The data shows that there was no difference in activation of the lats between the two exercises.
For “functional” strength purposes, wide-grip pullups are more effective as they employ biceps and lower back to a greater degree. However, pulldowns are superior for isolating the lats with minimal biceps and lower-back involvement. Thus, to get the benefits of both, start with pullups first, then move over to pulldowns. – FLEX