Why do all the human cultures that we know of decorate things? Why not just leave them alone? Why put in all that extra, and apparently non-functional, energy?

Why do all the human cultures that we know of decorate things? Why not just leave them alone? Why put in all that extra, and apparently non-functional, energy?
Links:
[1] http://www.edge.org/annual-question/what-is-your-question-why
[2] http://www.edge.org/inthenews/what-is-your-question-why
[3] http://www.edge.org/contributors/what-is-your-question-why
[4] http://www.edge.org/responses/what-is-your-question-why
[5] http://www.edge.org/print/response-detail/10876
[6] http://www.edge.org/memberbio/brian_eno