To replenish the chem's in our brain, balance them out...and also to replenish our energy...to reserve it...its like were very good and efficient rechargable batteries..

To branch out into the chemical side of it...say for instance you get drunk one night, and you wake up the next day with a hangover...well their are a bunch of alien chemicals effecting your brain and your judgement, and effecting other chemicals...so thats why you wake up kinda sick feeling and stuff because your body is trying to recharge those chem's that were effected and get the bad ones out of your brain and blood. (your system)