I'm sure many of us don't need a teacher to learn a specific math subject, just a good textbook. Self-studying allows us to learn at our own pace, rather than at the pace the teacher sets for the whole class, though the pace we choose for ourselves may not be the correct pace. We can also add/avoid topics of our own choosing, though we might not be making the best choice in so doing. People who self-study also usually don't give themselves assignments or tests (just trusting that they have learned well), which may or may not be a bad thing, depending on whether they truly have learned well or not. Of course self-studying is much more convenient, which thereby saves travel time and so gives us more time to learn.