Using a steady rest eliminates most vibration and chatter on thin spindles. When the wall on a bowl gets thin, it tends to vibrate and rough cuts are the result. A steady rest can eliminate that vibration and give you a much better finish cut. With taller vessels further away from the chuck, more leverage works against you — and the more likely you are to pull the vessel off-center. It doesn't take much to make a huge difference in concentricity.