Plants can not move or speak, but they still need to 'communicate' with their environment, including with other plants, animals, insects, and their specific pollinator(s). They also do not have an immune system; however, they need to defend against challenges from bacteria, fungi, viruses, insects, and even other plants.
How Do They Do This?
The primary way that plants accomplish both of these necessities - communication and defense - is through the production of "secondary metabolites" - compounds such as essential oils. (Other secondary metabolites include alkaloids (e.g.,caffeine), and even the pigments in the petals of flowers.)
Essential oils provide broad-spectrum protection to the plant against a wide variety of infectious threats in its environment. This is why essential oils can provide an equally broad-spectrum of antimicrobial action (antibacterial, antiviral, antifungal, antiparasitic etc.) when utilized by us humans! It also offers an explanation as to why an oil can have more than one action in the body, such as sedating, antispasmodic, and hormonal balancing, all at the same time: because essential oils are complex mixtures, not single chemicals. This complexity enables them to affect multiple pathways. Plants did not develop their essential oils to selectively target only one feature of an infecting organism (such as a synthetic antibiotic does); they actually connect with numerous physiological processes in the target organism.
Our metabolism is actually very similar, in many ways, to that of plants, including the production of strikingly similar molecules such as 'our' hormones and neurotransmitters. We forget that plants actually made them first! We have coexisted and coevolved with plants our entire existence, but plants have been around much, much longer than we have. If not for plants, we would not be here.
Something to ponder, the next time you stop to smell the roses!