For the major search engines, links to your website act as a vote. The text in the link and the quality of the page that links to your site are significant factors determining the importance of each vote. The more, and the better quality of links you have pointing to your site, the better your site will perform in comparison to search engines.
Good quality links also generate direct traffic. It is because link building is difficult, that search engines look favourably on well linked sites.
Gone are the days of link farms, or simple reciprocal links; link building today requires research and significant time to be completed successfully.
The quality of a page is determined by a Google Page Rank: the higher the rank, the more valuable the link, and the more difficult it is to achieve.