Bear with me here - I'm a domain name n00b.

How does the existing traffic to a site help someone who sells the domain name only - not the web content? Isn't content "king"? I assume the person who buys a domain name is going to create his/her own content. So why is there an assumed higher value if the former site had traffic? Won't that traffic go away immediately if the web content changes?

Curious how this works...

No, the traffic won't immediately go away.

Traffic = PPC for most people.

What's PPC?

Pay Per Click -> you sign up with a company like Fabulous and they pay you money when people click on links. Some of the biggest domainers do this for a living.

traffic can be manipulated in many ways - from ppc to redirects to affiliate programs to paid search bar downloads.... traffic is king.

companies like ultsearch make millions a year this way

Think of it this way, a homepage is kind of like a storefront that people can choose to enter or not. A store that sits on Broadway in New York City is going to be worth more than a store on Pecan Street in Pflugerville, Texas primarily because it has greater traffic.

More traffic = more sales = more profits = worth more.

Of course, then we get to the question of whether the traffic is quality traffic or not. In other words, if you acquire a domain name from an out of business baby store that is listed in DMOZ, yahoo, MSN, Google, etc. , you will have a better chance of making a sale if you put up another baby store website, or forward it to an existing baby website, or PPCs for baby related items. If you put up a porn site in it's place, you'll probably get alot of complaints to those search engine/directory sites that will get you dropped sooner... (and how could you sleep at night scaring all those new moms. ;) ).

Quantity in terms of traffic/eyeballs is less important vs quality of the traffic. You can have a domain name make 10X the revenue of a high traffic domain name with only a fraction of the traffic. This is a very competitive area where some very big fish play. This is why the secondary market(e.g. pool, snapnames) is growing.