With a spill, you can't be sure where the liquid has gone - it gets everywhere. Once it gets on the circuit boards, it can start to corrode the electronic circuitry. The idea of a deep cleaning (with purified alcohol or a similar cleaner) is to remove both the liquid and any residue the liquid carried in suspension. In the case of water, the concern would be minerals; in the case of beer, wine, soda or juice, the concern will be sugars, which can burn/caramelize with the heat of the circuit, and cause further damage. iFixit's [[Electronics Water Damage|Electronics Water Damage]] guide is a good start; also the [guide|296|MacBook Core Duo Logic Board Replacement] guide.