Google and other search engines such as Yahoo! or Bing use software programs called "Search Engine Spiders” to search the Web for content "relevant" sites. These spiders travel across links, then index and rank the web sites. Their rankings can be then seen on search engine result pages.
To be listed on top of the search engine pages it is critical to establish "relevant" link relationships, building both outbound and inbound links to your site. To help being ranked as high as possible it's necessary to build as many "relevant" links as possible.
The more "relevant" links you have the greater your chances will be to attract more visitors. It's an easy and very effective formula to boost traffic to your web site while it's also one of the least expensive ways.