Excerpt from: Don't Waste Time on Google AdSense
The biggest advantage of AdSense for domains is that you can earn advertising revenue from domains that you are currently not using. AdSense for domains is a way you can monetize your domain properties until you start producing content for your domain.
AdSense for domains help users find what they’re looking for by presenting relevant ads on unused domains. This aids users who type expired domains into their address bar or follow expired links leading to sites with no content. Usually, they would receive a 404 error or an "under construction" message. But with AdSense for domains, the user will receive links, search results, advertisements and other content related to the domain they were trying to access.
Not only does AdSense for domains help users get information relevant to what they are searching, AdSense for domains helps you monetize your unused domains by earning you advertising revenue whenever you generate page impressions and/or clicks on your domain’s Google ads.
Avoid common pitfalls and make money with Google AdSense by reading Don’t Waste Time on Google AdSense.