The constantly adding on to previous films' scenes is part of what sets Saw apart. This is a film series about a guy who was always a precise planner, in every aspect of his life. He basically didn't do anything until he had it planned out so he'd know it'd work. That's what all the adding on is about: we are introduced to characters and elements in this world that may seem unrelated to us at first, but either later in that particular film or the next one or the next, we see how John's plan was put into motion long before and that everything has played out as he had planned. If you don't get that, you're missing what makes these films truly unique.