Highly targeted, focused sites that are related to specific market segments are highly advantageous and can often be created using existing web content. The key is to provide value.
Think of the time spent surfing the web gathering resources and information. By creating a topic-centric resource compiling information, webmasters are providing a service or value. In many cases that value is simply the compilation of topic-specific information in a single resource. The compilation of this information in itself is the value.