During the distillation process, the steam takes along with it all the oil and water soluble compounds of the herb. The oil soluble compounds concentrate to create the essential oil of the herb. The water soluble compounds concentrate to create its hydrosol or flower water.
Within the plant itself, the essential oils have a protective role and ensure the wellbeing and propagation of the plant.
The essential oils kill pathogenic microbes. They also attract helpful insects that play an important role in propagating the herb and ensuring its expansion. They help to protect the plant by repelling harmful insects and they help repair damage to the plant itself. These processes in the plant are synergistic and involve hundreds of compounds. The unique combination of the compounds is extremely efficient and reduces the danger of pathogens developing resistance.
Humans learned to use these compounds for their benefit initially by experience and more recently through science. They help in fighting diseases and infections and in restoring and improving the function of the systems in the human body.
The pure essential oils contain the active compounds of each herb in high concentration. There are many ways to use essential oils. In order to be used safely regarding their dosage and method of use you should always seek advice. Hydrosols also contain essential oils but they are much less concentrated. Hydrosols are much gentler and can be used more liberally.