Buying an established Domain Name has major advantages in terms of traffic and search engine marketing for several reasons...

1. One of the many factors in Google's search engine algorithm is the age of a domain name.  The age of a domain name gives the appearance of longevity and therefore a higher relevancy score and a much better chance for a higher ranking in Google. 
2. An existing domain will already have traffic flowing in from various sources.  Obviously the amount of traffic will vary greatly depending on what the current owner has done to promote it, but any traffic is better than the zero visitors a fresh domain name will deliver.  Buying an existing domain name means you don't have to start from the bottom and build up from there.  You can use the existing traffic to grow your business faster and bigger than you could starting from scratch with a brand new domain name.
3. It takes time for a domain name to work it's way into search engine listings.  It can take weeks or months to get listed and even longer before a new domain name is given any type of recognition or ranking.  An established domain name already listed in the search engines will reap greater rewards as soon as you take it over.
4. Many of the good domain names have long been taken.  It is usually very difficult to find an unregistered domain name in a short, catchy phrase with a specific key word.