According to the Wellspring School for Healing Arts, mustard plasters are a traditional remedy used especially for clearing respiratory congestion. They have also been used in the past for gout and sciatica, with Herbwisdom.com claiming a general use for relief of muscle pain.
Mustard's healing properties are ascribed by the Wellspring School to its ability to promote blood circulation. When applied to the chest, the mustard plaster stimulates tiny blood vessels within the lungs, loosening mucus and easing the ability to breathe. Herbwisdom points out that mustard is warming when applied to the skin, which is why it is also used to relieve aching muscles.