가져다 is composed of
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.
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."