The Wort then needs to be cooled down as quickly as possible. There are a few reasons:
During the boil, proteins in the wort clump together and form complex structures. Rapid cooling helps the proteins and other stuff created during the boil to separate out (become insoluble) and sink to the bottom. This results in clearer beer.
It helps keep the aroma oils in the brew, so the final beer has a hoppy aroma.
The quicker the liquid cools, the quicker the yeast can be added and reduce the chance of any contamination from unwanted airborne microorganisms from the environment.
When the Wort is still warm (above ~26°C), oxygen can interfere with the alcohol by oxidation which can result in an off-smelling beer, which no one wants!
