Gravity will not allow you to stay the same

The more you use your body as it is designed to be used, the better your posture will become and the better it will feel. Conversely, poor use invites deterioration. You can get better or you can get worse, but gravity will not allow you to stay the same.

Mary Bond "The New Rules of Posture" page 12

A balanced posture generates alert awareness

A balanced posture requires very little effort to sustain and allows the major muscles of the body to relax. This relatively small expenditure of energy, coupled with the phenomenon of relaxation, produces a distinct feeling tone of softness, ease, and vibratory flow. It also generates a natural condition of alert awareness. This dual condition of comfort in the body and relaxed alertness in the mind is the fruit of balance.

Will Johnson in "The posture of Meditation" page 20

Posture

Posture is the position assumed by the body either by means of the integrated action of muscles working to counteract the force of gravity, or when supported during muscular inactivity. Many postures are assumed by an individual during the course of 24 hour, and at any given moment posture comprises the positions of all the joints of the body.
Dynamically and statically, an efficient posture:

1. is stable
2. minimises stress and strain on the tissues
3. minimises energy cost

Subtle departure from an optimal posture is common and subjects the body to routine, eccentric and increased mechanical stress.

Alison Middleditch & Jean Oliver "Functional Anatomy of the Spine" page 327