The more sites that link to you, the more traffic you can expect to receive to your site. People find nearly as many Web sites by following links from one Web site to another as they do by using the search engines. Higher search rankings, of course, translate into greater traffic. The more links to your site, the more ways search engine spiders have for finding you each week. Therefore, you tend to stay indexed longer and are less frequently dropped from the index.
Some engines favor links from popular sites.
Therefore, a few links from Web sites with a high link popularity score may be given greater importance than a larger number of links from less popular sites. There's various banner exchange networks out there that have a network of linked sites. They generally link to their own Web site and then redirect to the intended destination in order to record the click through or impression. These links often contain CGI parameters which many search engines refuse to follow at all. Therefore, don't expect your link popularity score to improve with those types of link exchange services.