Physics:

The coolness condenses the smoke, basically concentrating it. You can suck in a whole lot more smoke that non-cooled smoke. It expands from it's condensed state in your lungs, instead of harshing your throat all the way down. Throw your bowl in the freezer every now and then, that's good too.

It's actually best to tip the bong up when you are clearing it (when doing the ice idea). Some of the condensed smoke will be heavier than the lower pressure air and sit in the bottom of the bong... just give er a tippin and it will flow with a viscocity similar to water