First off, linkbuilding is the process of submitting one's link or articles to directories, buying or exchanging links and participating in forums/discussion boards/blogs. Sites that undergo this process intend to promote their sites and obtain links from other worthy sites to increase traffic to their site and ultimately gain a high page rank in search engines. In addition, links shows a site's credibility and offers more opportunities in the long run.

Linkbuilding is a never-ending endeavor but the online world is huge so it takes some time to be noticed and be visited by people. Thus, constant exposure will not only bring in people but revenue as well especially if your site sells products.

Linkbuilding helps a site gain value because the more links that a site has, the more credible it becomes. Keep in mind that one way links are better because it is ranked higher by search engines.
Lastly, linkbuilding establishes connection with the online community. More opportunities present themselves to companies through this such as referrals, more business contacts, paid seminars or training.