A bag has only one purpose. To protect what's inside and to keep it in one place. If you only have a few items, or just one, that bag could be a waste of time. Those who have something like 15 or 20 items may wish to carry a bag so they can carry it all without having it fall apart.
Style? Anything that will get the job done. If your bag is too big, you might be pressured into increasing your field circus, as a big bag holds more littera-trash than a smaller one. Nylon or canvas is acceptable, since leather can get ratty looking in a few years, and it holds water (while nylon will let water in, it will also let it dry quicker. With leather, once water gets in, it is stuck in).