Though all vegetables are important for health, certain kinds offer unique benefits. Onions are members of the Allium genus of flowering plants that also includes garlic, shallots, leeks and chives. These vegetables contain various vitamins, minerals and potent plant compounds that have been shown to promote health in many ways. In fact, the medicinal properties of onions have been recognized since ancient times, when they were used to treat ailments like headaches, heart disease and mouth sores.