

So, in the opening of The Fellowship of the Ring, that Gollum was once a hobbit named Sméagol is an extraneous detail.

It is a bit of a shocking scene the shock would not be nearly as strong if the viewer already knew Gollum's origin. The Return of the King opens with the story of Sméagol and Déagol to shed new light on who Gollum is, where he came from, why the Ring means so much to him, and what kind of influence the Ring wields over him. In The Two Towers, he has a much larger role and seems sort of halfway between beast and person, but a viewer who has not read the books is still unlikely to guess that he is a corrupted hobbit.

In The Fellowship of the Ring, the viewer is intended to see Gollum as a sort of beast. Peter Jackson didn't want to tell the story of Sméagol until after the character Gollum was established. I think it was worded that way for pragmatic reasons.
