Is somehow the same as Blog Catalog. The idea, is to be able to have a widget on the sidebar,  to know who visited your site, without the visitors leaving any message.

This is also good for marketers, to know bloggers or blog that is about a specific topic. You could check the community and have huge list of bloggers within that said topic. There you could get a list of people you would like to get in touch with while promoting a site.

This is owned by Yahoo.