in short - stimulating the upper back, above the lats vs hitting the lats more directly.
in long - There is a thread from a while ago in t-cell discussing theory and truth about wide grip for a wide back. it's titled why is bob cicherillo is a s asshole or something like that...it's my old thread.
the back needs to be hit from a myriad of angles addressing width and thickness, horizonal vs vertical plane movements and variances in between those two angles.
in agreement w/ stu wherein it's not so much the width of the grip but what individual choice movement pathway allows each individual to address and maximally stimulating the lats and above e.g. upper back -rhomboids and teres major, these muscles create width along the upper back.
speaking of that, learning to properly retract the scapula and then spread your lats and back just like a bodybuilder would in a rear lat spead, this is a great way to help w/ back training. if you cannot flex and control your back you'll have a more difficult time determining the gray areas of different back training exercises and/or knowing when one muscle group is taking over a movement intended for another muscle group. this occurs commonly with rear deltoids.
wide grip movements shorted the range of motion and cause more stimulation in the center point of the upper back. wide grip while hanging creates the stretch feeling in the back, which is good but often mistaken for what it is not - a contraction. a stretch and contraction are felt but in different places.
the bottom line, w/ variance in exercise, and the slight to great variance in form, limb size, bla bla bla, the back grows wider and thicker. it isn't pulldowns for width and rows for thickness absolutely, but staple movements create the lion share of growing.
illustration of the point - i've never seen anyone w/ huge triceps who got them by doing kickbacks as the their primary mass builder.