The onion family works on three levels. They can be considered a vegetable, a herb or purely as an ornamental plant. They are amongst the oldest plants grown as vegetables and have changed relatively little in thousands of years.
The various ornamental onions, with their distinctive rounded flower heads, make striking plants for lots of different situations. Some onions are no longer known in the wild and their origins are obscure. Garlic no longer sets seed and apart from a few modern mutations has been grown in the same varieties for many generations.
One advantage of the age and widespread nature of the onion family is that there are always some that can be grown in any climate and soil condition. Another advantage is that they are generally very easy to propagate. They produce plenty of seed as well as multiplying vegetatively by producing masses of small bulbs. In some case, they also produce bulbils in the flower head.
For some the main disadvantage is that the foliage can be unattractive and often foetid (although there are some sweet smelling onions). This is a particular nuisance with ornamental onions where the foliage is already dying as the plant comes into flower. One way of coping with this in a ornamental garden is to grow the onions amongst other plants so that their foliage is masked.