Because when that child comes back and hands in that work the teacher has to correct it. She/he will have most likely have moved onto a new lesson, working on that and correcting papers for that one. Now she has to make sure Jr. has all his stuff from the previous week done, and needs to make sure Jr. understands the lesson he missed because he'll be behind the rest of the class if he doesn't. It wouldn't be "extra" had he been in class when they were working on it, but he wasn't so the teacher has to "work" on him on top of her current lesson. Do you think teachers just put papers in piles?