Plantar Fasciitis
Plantar Fasciitis is essentially a tendonitis.  The arch of the foot is supported largely by a wide band of connective tissue along the underside called the plantar (bottom) fascia (connective tissue).  Tension thru the band helps keep the bones of the feet from falling to the ground.  Pronation (rolling "in") or supination (rolling "out") of the foot changes the tension thru the band as well as the position of the bones it supports and connects. If you habitually roll your feet- because of poor posture, weak foot muscles, unbalanced calf muscles, etc- this can stretch the fascia. 
While some stretching of the fascia is inevitable with use, continual stretching can cause the body to react as to an injury.  Swelling occurs, and the body starts laying down adhesions to support the 'injured' area.  Repeating this process over and over lays down more and more adhesions which eventually cohese into scar tissue.  Scar tissue is not as flexible as connective tissue, and is much more sensitive.  Pain begins to affect the gait, and this causes even more swelling as the fascia is stretched in a new direction.  In extreme cases, bone spurs begin to form as tension affect bone growth, and the body attempts to shorten the fascia itself. 

Why massage?
Massage can help plantar fasciitis in several ways.  Relaxing and balancing the calf muscles can reduce pull on the bones of the feet and help relieve stress on the fascia.  If scar tissue has already begun to form (as is usually the case when pain causes the need for treatment), massage can help break it down.  This reduces sensitivity and helps realign the bones. Postural realignment of the entire body can also help reduce pronation or supination of the feet.  Massage can NOT remove bone spurs, but once the fascia is relaxed, bone spurs will stop growing.