These links or bookmarks are the tagged pages on the web that can be accessed from any computer with internet access. Thus, every such bookmark acts as a backlink that helps improve the SEO of a site.
Importance of bookmarking:
Social bookmarking sites make it easy for people to find websites and help you generate backlinks to your website. If you bookmark your website more in social bookmarking websites, you will get backlinks from each website. This results in a higher page rank for your webpage or website.