Let me start by saying the movie was really well paced. The Barrens played a really large part in the book but was mainly absent from the film. At 2:15, If the Barrens, the clubhouse/smokehole, the damn, and like arcs were included, would it really have added anything to the experience or just been too long.
shareI think it would because the Barrens really set the tone of the mini-series. They provided a claustrophobic atmosphere that in itself was almost like a character, because it told you a lot about IT without having to spell things out; and also about the kind of freedom kids had to roam in dangerous places without parental supervision, despite the fact kids were dying and going missing.
Also, I think it added to the mystique of IT. Kids often have their own secret places that they retreat to, away from adults, and violating that by making it a monster's lair really kind of upped the fear factor.