Originally Posted by Libertine

When someone searches for the terms in a domain name, he is presented with thousands of links to check out. The site in question is one of them, but so are torrent sites, tubes, gallery listings, filesharing forums, etc.

Unlike se listings, using adwords gives you full control over how exactly the link will look - creating the opportunity to not only increase chances of a click going to the site in question rather than a tube or torrent site, but also possibly enticing the user to consider joining the site instead of continuing to search for free stuff.

Compare it to a billboard right in front of the store it advertises. Everyone who passes it should notice the store as well, and yet it is likely to still bring in additional customers.