Each element performs many functions
By carefully selecting elements that compliment each other, it is possible to ensure elements can peform multiple functions. For example; "A dam wall functions as a road, a firebreak and a bamboo production area." Mollison and Slay.
The consideration is specific to the element you are choosing. If you are choosing a plant, it is important to consider its needs, its tolerances and its products.
Image: David Collier, "On the road up from Cwmtydu (pronounced 'coomtoodie') beach." A hedgerow can perform many functions, including: a windbreak; habitat for galls, lichen, insects and birds; food for human and animal forage; a boundary marker; a stockproof barrier.