Hmmm...I always thought of the Overcouncil as a "sort-of-United Nations" of Erebus with more or less the same goals as the real life UN have (at least in their Charta):
Peace and human rights (yeah, that's a simplistic and somewhat idealized summary, but hey ^^). And since we are in Erebus also combat against the forces of evil (though the way of fighting certainly depends on who is in charge...)
However, the Overcouncil appears to be a lot more (directly) active than the UN.