Similar to the bench press, different width grips work different areas. A close grip will work the inner back muscles: traps, romboids, and inner-lats. Those muscles provide thickness. The wide grip will work the outer back muscles: Teres major, infraspinatus, and, to a certain extent, the rear delts, which will provide width.
Personally, I like a wide grip because I feel the seated row is better excercise for working the inner-back.