In China the first recorded use of Ginger was by Confucius (circa 500BC) who claimed never to be without ginger when he ate.
In Traditional Chinese Medicine ginger is used for colds and chills to promote sweating, expel mucous and stimulate the appetite. The dried ginger has been used to treat stomach ache, dihorrea, nausea, cholera and bleeding (Leung A, Foster S Encyclopedia of Common Natural Ingredients used in foods, drugs and cosmetics).
As regards the essential oil it is steam distilled from the dried rhizomes.
The scientific approach is that if you understand the main components of the oil then you will know the general effects. However particular uses of essential oils depend on components which may be present in very tiny quantities. The main components of the essential oil are sesquiterpene hydrocarbons (50-66%), oxygenated sesquiterpenes. If three isoprene units join they make a longer and heavier chain molecule known as a sesquiterpene (15 carbon atoms). Sesquiterpenes are slightly antiseptic, bactericicidal, calming and anti-inflammatory. (Price, Aromatherapy Workbook).
Essential oil should always be used in tiny quantities and never be taken internally but applied externally in diluted form or inhaled.