In general, I think the core idea that makes biological brains understandable is rewards. The following "Reward Hypothesis," as described on the RLAI research group's site (http://rlai.cs.ualberta.ca/RLAI/rewardhypothesis.html), summarizes my point:
"That all of what we mean by goals and purposes can be well thought of as maximization of the expected value of the cumulative sum of a received scalar signal (reward)."
I think this is a clear description what the brain is "trying to do." All other major brain functions make the task of reward maximization easier.