The oil from Eucalyptus trees provides several benefits with the most prevalent being the pleasant aroma.  As a tree that grows fast and has been used strategically to reduce the risk of malaria by draining swamps a little Eucalyptus goes a long way.  The oil produced by these trees are also useful in an antimicrobial activity and in some cases has been known to make the trees more flammable.  Outside of their native Australia they have been viewed as an invasive species.