I imagine its part of the 'civilization' package. More fuel-efficient, maturing to a more juvenile form, less aggressive, and more specialized brain? In a community you only have to learn a few specialized tasks, instead of needing to have all the skills?
Living in a dense village/town/city its important to be efficient especially with food. Its the fundamental limit on scaling. We don't all have to be warriors. Some sit and make clay pots or weave. A smaller, fuel-efficient model of citizen is a great advantage to a society.
