Bobbin weight threads are typically 60wt, so thinner than what we use to quilt with (typically). The main reason people favor them when quilting is because more thread fits on the bobbin, therefore less starting and stopping.
Saying that, you can quilt with anything, in both the upper thread and in the bobbin. Some people like 100wt silk for intricate quilting, while others prefer a very chunky 17wt thread for high definition.