Those other oils which claim to have medical benefits have historically been taken internally. The application of these oils to the skin has no effect whatsoever. Aromatherapy, like other alternative medicines, works only as a placebo. All so-called medical benefits are through belief rather than any direct action. The action of essential oils is psychological rather than physiological.
Essential oils represent the 'life force' of a plant.
Steam distillation is often promoted as the best way to extract essential oils but this process removes many of the active ingredients. Cold expression is a far superior method for many oils. Most aromatic oils used prior to about 1600 AD were not distilled, but were produced by macerating plants in hot vegetable oils or more commonly in animal fats. The resulting product contains a similar amount of essential oil to that found in a modern diluted massage oil. However, the end result in terms of chemical composition is very different.