It's fairly important for several reasons:
DMS, a cooked corn/vegetable off-flavor, is produced while the wort is still hot. During the boil, DMS simply boils off, but while cooling it's advisable to minimize the amount of DMS produced. The yeast can clean up some DMS, but the less you have to begin with, the better.
Sanitation, the longer your beer is exposed to air and other environmental factors, the higher the chance for contamination from bacteria or wild yeast. I believe when the temperature is < 140F, the wort is at the most risk.
Another reason is to form a good cold break. Cold break is coagulated protien-polyphenol compunds. It's my understanding that a faster cold break promotes more coagulated proteins and reduces the chances of beer haze.
And of course if you value your time, you'll want to chill quickly. Before I got an immersion chiller, it would take forever to chill my wort using ice baths. Plus, I was buying ice just to melt it while chilling my wort. To me, a wort chiller just made sense in order to save time, money, and produce better beer.
I hope this helps. If you're planning on, or are already brewing 5 gallon full boils, I'd strongly recommend using a chiller. It makes life so much easier and your beer should benefit based on the reasons mentioned above.