I've been in a long and involved thread on grip width for pulldowns, chins and variants. The conclusions reached were basically explained by noting the position of the humerus bone in relation to the different grips. With the mythical wide grip (the one most people hear is for 'width') leaves the humerus straight out to the sides. To do a narrow grip, you have to extend the humerus upward , pulling the lats up into a longer ROM, and a different angle of attack in the beginning of the movement.
As you do your pull, the humerus moves through the extra plus the prior ROM, accomplishing almost the same thing, but more.
I say 'almost' because there is a slight difference in the position of the elbows, narrow grip requiring them to be slightly more forward. But that is a good thing for involving the upper lat.
also try holding your hands above your head shoulder width apart, you wont feel a pull on your lats, now try touching your hands you will feel the pull,so IMO close grip is better for stretch and it allows you to use more weight and there is less chance of injury to shoulders and bis.