It is essential to cool the wort relatively quickly for a few reasons.
1. IBU: When adding hops late in the boil, temperatures above 45°C allow for continued isomerisation of the alpha acids in hops. Therefore, slow cooling may result in a significantly higher IBU than intended. If you use the Grainfather app, then the mIBU software will consider your cooling method for a more accurate prediction.
2. DMS production: many malts, especially under modified malts, contain DMS precursor. One of the goals of the boil is to ensure all this precursor is boiled off. However, with slow cooling times, this precursor can form, resulting in DMS. If cubing, it is best to ensure long boil times and use more modern modified malt.
3. Contamination: below 72°C, your wort is susceptible to microorganisms. The longer the open wort spends open to the environment below this temperature, the higher the chance of contamination from wild yeast and bacteria.