Muscles contract for reasons unrelated to weight-bearing.  Tightening the body is an automatic response to stress or danger. During the stress response our breathing gets shallow as the muscles of the abdomen, ribs and diaphragm contract. 
Other components of the stress response may include tightening of the muscles of the shoulders, neck, jaw, even the arms and hands, legs and feet. 
These contractions are a chronic energy drain. People who are not in touch with their bodies, may not be aware of these contractions until the muscles begin to ache or cramp, a signal that blood circulation in the muscles is insufficient. 
Chronic, long sustained tension eventually causes the connective tissue around the compressed parts to shorten.  The body has become disorganized.  When the person later attempts to correct his posture by lengthening the area,  he must do so by using muscular effort to overcome the resistance of the shortened connective tissue.
Simply put,  a disorganised body can maintain good posture only by straining, whereas an integrated body can maintain good posture and be at ease.
Rolfing can lengthen and release shortened connective tissue, improve blood flow, and eliminate energy losses caused by the strains and chronic muscle contractions in a disorganised body.