If you get a roll, you can cut it to your desired length and then just seal up the bottom, and top (after you put your stuff inside). These ones are extremely versatile, as well as usually being a bit cheaper. Some bags come pre-cut, in which case they'll come sealed on 3 sides. Some people prefer the convenience of these pre-cut ones. However, they often end up being more expensive, particularly when you consider that you may be using a bag that's too big for your actual needs.