Why do eucalyptus trees have such an excellent aroma?  Is there any other uses for Eucalyptus?

Context: Some Eucalyptus species have attracted attention from horticulturists, global development researchers, and environmentalists because of desirable traits such as being fast-growing sources of wood, producing oil that can be used for cleaning and as a natural insecticide, or an ability to be used to drain swamps and thereby reduce the risk of malaria. Eucalyptus oil finds many uses like in fuels, fragrances, insect repellence and antimicrobial activity. Eucalyptus trees show allelopathic effects; they release compounds which inhibit other plant species from growing nearby. Outside their natural ranges, eucalypts are both lauded for their beneficial economic impact on poor populations: 22  and criticised for being "water-guzzling" aliens, leading to controversy over their total impact.

Answer: 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.