When you tear the fabric, it kind of "bruises" the edge of the fabric, and it must still be cut, it gets kind of ruffly and makes a soft fringe, but it is useful if you have lots of yardage that you want to cut some off from. I often buy a bolt of black, and tear off just a yard or two when I want some.
If you decide to tear fabric, make a small cut, and tear fast! You will have to tug on it twice, but if you go slow, it seems to make the fabric stretch worse on the torn edges.