If you think of a situation where a singer is performing with a band, it would look fairly dull if the singer stood straight in the one position all the time. Movement is a large part of any stage show.
This means it is not always possible for the singer to maintain perfect posture. However, it is possible to keep the pathway from the diaphragm to the mouth open, flexible and relaxed most of the time, which means it is still possible to sing well while moving around. Relaxation and flexibility are keys to good posture, regardless of standing or sitting position.