- aerodynamic advantage
- protection from predators
- communication - they can see one another better
- expresses social relationships - pecking order, dominance
Many people seem to think there should be a single correct answer to this kind of question. But POSIWID doesn't make this assumption. Instead of one strong and self-sufficient purpose, we may well find two or three weaker but mutually reinforcing purposes.
In order to prevent this misunderstanding, perhaps we shouldn't gloss POSIWID as the purpose of a system is what it does but rather a purpose of a system is what it does.
See also Constructing POSIWID (April 2022)