It's important to rapidly cool your wort for a few reasons.
1) Once the beer falls below the pasteurization temperatures (170-180), it is susceptible to contamination. Cooling it quickly and racking to primary minimizes the time it is exposed to open air.
2) Helps break material fall out of suspension in your primary fermentation vessel
3) Stops hop acid isomerization if you use whirlpool hops, keeping your beer from getting too bitter and/or destroying delicate hop oil aromas.
We recommend a counter flow chiller or plate chiller to be hooked up between your boil kettle and primary fermentation vessel. This is the quickest way to chill your wort and keep it from getting contaminated.