가져다 is composed of 가지(다)+어다 어다 is a connecting particle used for describing sequential action. 어다 is a short form of 어다가 어다 is usually indicate movement of place between two actions. ex) 김밥을 사다가 먹었어요. (김밥을 산 장소 <> 먹은 장소) I bought gimbap at a store and ate it at some other place. 도서관에서 책을 빌려다가 읽었어요. (책을 읽은 장소 <> 도서관) I rented a book at a library and read it at some other place. 그는 책을 가져다 왔어요 means "He took a book at some other place and brought it here." 그는 책을 가져왔어요. means "He took a book and brought it here. I'm not sure whether the book was here or some other place."