There are many ways to promote your site. And posting backlinks is one of the most effective methods I know of. But what if you need to link to a site with strict moderation? A comment with an advertised link is usually immediately removed by moderators from such a site. Then you can place a link to a more authoritative resource, for example, to an article in a popular blog, where the advertised link will be placed. That's what multi-level link building is all about.